from the it-seemed-like-the-logical-thing-to-do-at-the-time dept.
A couple of months ago, it was a color-changing dress that blew out the neural circuits of the Internet. Now Kenneth Chang reports in the NYT that a problem from a math olympiad test for math-savvy high school-age students in Singapore is making the rounds on the internet that has perplexed puzzle problem solvers as they grapple with the simple question: "So when is Cheryl's birthday?"
Albert and Bernard just met Cheryl. “When’s your birthday?” Albert asked Cheryl.
Cheryl thought a second and said, “I’m not going to tell you, but I’ll give you some clues.” She wrote down a list of 10 dates:
May 15 — May 16 — May 19
June 17 — June 18
July 14 — July 16
August 14 — August 15 — August 17
“My birthday is one of these,” she said.
Then Cheryl whispered in Albert’s ear the month — and only the month — of her birthday. To Bernard, she whispered the day, and only the day.
“Can you figure it out now?” she asked Albert.
Albert: I don’t know when your birthday is, but I know Bernard doesn’t know, either.
Bernard: I didn’t know originally, but now I do.
Albert: Well, now I know, too!
When is Cheryl’s birthday?
Logical puzzles like this are common in Singapore. The Singapore math curriculum, which has a strong focus on logic-based problem solving, has been so successful that it's been adopted around the world. According to Terrance F. Ross, US students have made strides in math proficiency in recent years, but they still lag behind many of their peers internationally, falling at the middle of the pack in global rankings. In the same PISA report the U.S. placed 35th out of 64 countries in math. "And even though the "Cheryl's Birthday" question may be atypical of the average Singaporean classroom, perhaps it's still worth asking: Are you smarter than a (Singaporean) 10th-grader?"
(Score: 4, Funny) by Anonymous Coward on Thursday April 16 2015, @01:30AM
This is the Police. Cheryl, let's see some ID.
(Score: 5, Funny) by sigma on Thursday April 16 2015, @01:36AM
Fuck off Cheryl, if you want to be a bitch, you're not getting a birthday present from me!
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:42AM
Go, go, go, go
Go, go, go Cheryl
It'z your birthday
We gon' party like it'z yo Birthday
We gon' sip Bacardi like it'z yo Birthday
'Cuz you know we don't give a fuck
It'z not your Birthday!
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @05:11PM
No birthday present for you on May 19th.
(Score: 2) by kaszz on Thursday April 16 2015, @01:36AM
Always ask, is this a problem worth to bother you neurons and time with?
(Score: 4, Funny) by Anonymous Coward on Thursday April 16 2015, @01:38AM
These grapes are sour.
(Score: 5, Insightful) by Tork on Thursday April 16 2015, @01:42AM
🏳️🌈 Proud Ally 🏳️🌈
(Score: 1, Disagree) by Anonymous Coward on Thursday April 16 2015, @01:50AM
He deduced, correctly so, that this dumbass problem is a waste of his time. His deductive skills far exceed yours, clearly.
(Score: 3, Informative) by Tork on Thursday April 16 2015, @02:36AM
He deduced, correctly so...
No, he didn't.
🏳️🌈 Proud Ally 🏳️🌈
(Score: -1, Troll) by Anonymous Coward on Thursday April 16 2015, @02:38AM
Yes, he did.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:51AM
Mmm hmm. Captain Correct they'll call him.
(Score: -1, Offtopic) by Anonymous Coward on Thursday April 16 2015, @03:07AM
He intelligently deduced that he needn't waste his time solving such useless problems when there are far more interesting, difficult, and useful problems to be solved.
(Score: 1, Funny) by Anonymous Coward on Thursday April 16 2015, @03:51AM
(Score: -1, Troll) by Anonymous Coward on Thursday April 16 2015, @04:04AM
Bitching about the stupidity of this idiotic problem is much more useful than trying to solve it.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @04:05AM
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @05:49AM
No one says you can't have free time.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @07:08AM
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @10:15AM
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:17PM
There was no self-defeating argument; he obviously decided that riddles such as this are a waste of time.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:35PM
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @07:41PM
Then:
"I intelligently deduced that I do not need a useful skill!!"
Maybe he does not find it useful (as another commenter says, this really only tests a specific skill that will likely almost never come up in the real world outside of some artificial environment), or he already solves far more complex problems and doesn't feel the need to solve poorly-written logic puzzles in his free time, but at the same time, wants to do other things in his free time.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @09:13PM
(as another commenter says, this really only tests a specific skill that will likely almost never come up in the real world outside of some artificial environment)
That was refuted.
Maybe he does not find it useful [...] or he already solves far more complex problems and doesn't feel the need to solve poorly-written logic puzzles in his free time, but at the same time, wants to do other things in his free time.
Or he rushed to dismiss it because it's mainstream and now he's trying to save face.
(Score: 3, Interesting) by tftp on Thursday April 16 2015, @02:09AM
You don't think exercising your deductive skills is worthwhile? We must work in very different professions.
Indeed, your profession has to be fairly unique if you solve puzzles like this for living. The remaining 99.999% of the population don't solve riddles - they drive trucks, they dig trenches, they run machines, they write WPF GUI code, they do many other things that have nothing to do with anything like this.
Those 99.999% do solve some problems at work - but these are not riddles like this example. They are more complex. How to build this retaining wall so that it won't fall down? How to drive from A to B on these roads, considering their quality and the probability of meeting a cop with unmet quota? How to mill this piece without incurring too much vibration and tool wear? How to design this amplifier so that it won't oscillate? Answers to those professional questions come from education and from experience, and that's what those workers are paid for.
This puzzle is not even a complicated one; all you need to do to solve it is to write down the dates and then emulate knowledge of both A and B, step by step. As you do that, you understand that there is only one solution that makes answers of A and B possible.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:38AM
Bad news. HR just decided to make a brain teaser part of your next performance evaluation. When you fail because you weren't taking the puzzles seriously enough, you will be fired and replaced with cheap obedient Singaporeans.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:42AM
Bad news. Blowjobs and the ability to draw a circle freehand just got on the list too.
(Score: 5, Insightful) by Tork on Thursday April 16 2015, @02:48AM
Indeed, your profession has to be fairly unique if you solve puzzles like this for living.
Maybe I do have an unusual profession, but I doubt it. I've had a number of jobs and even when I worked in retail there were times where deductive reasoning has been helpful. I think you summarized that really well right here:
This puzzle is not even a complicated one; all you need to do to solve it is to write down the dates and then emulate knowledge of both A and B, step by step. As you do that, you understand that there is only one solution that makes answers of A and B possible.
This sort of rationale applies to inventory, poor communication from a customer/client, reading a form that has poor handwriting on it, video analysis, interview at Google, etc. Sorry, I really don't get all the negativity with this one. Unless, of course, this isn't about the value of the puzzle at all and is instead just general begrudgery against anything mainstream.
🏳️🌈 Proud Ally 🏳️🌈
(Score: 2, Disagree) by tftp on Thursday April 16 2015, @03:03AM
Sorry, I really don't get all the negativity with this one. Unless, of course, this isn't about the value of the puzzle at all and is instead just general begrudgery against anything mainstream.
The puzzle is terribly contrived. People are reading crime stories where the plot is much thicker than that - and they enjoy those stories, as they are realistic. People can imagine themselves in the role of the detective. It all starts with a loosely defined problem. Then the protagonist tries to obtain more information - which appears to be contradictory, so someone is lying. He figures that out and gets one step closer to the solution.
The very first post here approaches the solution in a far more practical manner. We do not deal in riddles. Most people get angry when someone is toying with them. People want clear and specific information; they don't like to guess. It is not even practical to guess, as reliability of the solution becomes dependent on the fuzzy input. And, finally, not everyone is a mathematician. I recall an annoying mission in one of GTAs where you had to press various buttons on the controller to follow a dance tune. I am not capable of that. At some point I was thinking of connecting a GPIO to the controller and programming the button presses, as for the life of me I could not get through that mission. But eventually, after many tries, I just memorized enough of the pattern to pass.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:25AM
Manipulation is how people interact with each other. Don't like it when everyone is toying with you all the time? You are the problem, and we have an anger management class to fix you. If you don't learn to enjoy being lied to, you're fired.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:44AM
Not liking to be toyed with was itself a lie.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @10:56AM
I'm guessing you're not from Singapore, then.
If you think this story has *anything* to do with guesswork, then you're *really* not the intended audience for logic puzzles.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:19PM
It's guesswork because you have to make all sorts of assumptions to hopefully answer in the way they wanted you to answer. See a bunch of other replies on the assumptions you have to make.
(Score: 3, Funny) by tftp on Thursday April 16 2015, @05:21PM
It's actually worse than that. Your answer hinges on ALL OF THE RESPONDENTS correctly analyzing the problem and providing you with correct replies. This magnifies instability of the solution.
Imagine the following exchange at work:
Boss: Listen, John. We have a customer who is willing to pay us $50M for a job. I need you to figure out how much that job will cost us, so we can bid or no bid on it.
John: OK, boss, I'm on it.
John: Hey, Jane, how much this will cost us?
Jane: John, it will be between ten and a hundred million, but it's an even number. Ask Jill for more.
Jill: John, it will be not a prime number. Ask Will for more.
Will: John, if you square this number and then calculate a factorial, it will end in two. Ask James for more.
. . . . . .
Boss: So, John, what is it? I'd hate to lose our shirt on this contract if we make a mistake.
John: Boss, I talked to five hundred and thirty seven people in our company. None of them gave me a specific answer. All I have is a bunch of vague hints. I summarized them in this here spreadsheet. According to my calculations, if I am correct, the answer would be 37 million, and we will make a reasonable profit on it. However three hundred and eighty two answers are the keystone of this solution - if they are wrong then our costs could be anywhere from one million to three hundred million. I hope, boss, that this answers your question.
Boss: @%$#!!!
(Score: 1, Interesting) by Anonymous Coward on Thursday April 16 2015, @08:20AM
This sort of rationale applies to ... poor communication from a customer/client
No it doesn't. In too many cases you cannot use this sort of logic when you get poor communication from a customer/client.
That's because lots of customers and clients do not think clearly and logically. So they might send you some garbage of an email.
Trying to use logic on the garbage they send you often leads to wrong conclusions (GIGO).
So when you detect some ambiguity what you have to do is work out a way to get them to send something hopefully clearer. Sometimes it's simple as just asking, at other times you need some "people sensitive" way of doing it.
Just going "But you said this in your original email and so we'll hold you to that otherwise see our lawyers" might be fine if you only want to do one job with that sort of customer (and they are rather common- after all in many cases the clear thinking ones might not need you as much as those with candy floss for brains).
(Score: 2) by Tork on Thursday April 16 2015, @03:47PM
No it doesn't. In too many cases you cannot use this sort of logic when you get poor communication from a customer/client.
And how exactly would you determine if you do not have enough information from them? ;)
🏳️🌈 Proud Ally 🏳️🌈
(Score: 1) by tftp on Thursday April 16 2015, @11:23PM
And how exactly would you determine if you do not have enough information from them? ;)
You determine that by talking to them and figuring out if they understand what they are asking you to do. Plenty of customers have unrealistic expectations. It would be a disaster to start a job only to shock the customer in the end. You always want to ensure that the customer knows how much is he going to pay and what is he going to get - before you even start. The customer may be silly and illogical, but that won't prevent him from suing you. You cannot depend on hints - you always want everything important clearly spelled out and clearly understood by all parties.
(Score: 2) by Tork on Thursday April 16 2015, @11:59PM
🏳️🌈 Proud Ally 🏳️🌈
(Score: 1) by tftp on Friday April 17 2015, @12:17AM
I'm saying that most communication is poor, and you try to make at least the essential communication - that can make or break your business - as clear and obvious as possible. It may well be that you will be listening to your contract being read aloud, in court. You want to make sure that you made all the effort that you could to deliver your message, and that it's not your fault that the other guy still managed to misinterpret it.
Obviously, if the contract contains confusing messages that need to be read by three lawyers and five mathematicians from Singapore, this won't fly real well in court even if they are mathematically correct, like this riddle. Can you imagine that the girl would be giving answers like that to two police officers? Or that they will even attempt to figure out the answer?
(Score: 2) by DeathMonkey on Thursday April 16 2015, @06:06PM
This puzzle is not even a complicated one; all you need to do to solve it is to write down the dates and then emulate knowledge of both A and B, step by step. As you do that, you understand that there is only one solution that makes answers of A and B possible.
Basically, process of elimination.
I'm sure nobody on this website has used the process of elimination while troubleshooting something.
How to mill this piece without incurring too much vibration and tool wear? How to design this amplifier so that it won't oscillate?
No troubleshooting to be seen here, move along.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:46AM
You are dumb. Muahahahaha. No, not because you can't solve this puzzle, but for whining about it.
(Score: 2) by c0lo on Thursday April 16 2015, @02:53AM
In my case the answer is: if I like the problem, I'm going to do it.
Bickering problem solved! Next? Ah, TFA problem? Well July 16, of course - resembles Sudoku, with some custom rules.
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:14AM
Yes, it's more worthwhile to bicker about how fucking useless this goddamn question is than it is to try solving it.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @07:51AM
Always ask, is a comment like yours worth to bother your neurons, finger muscles and time with?
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @01:36AM
And I'm not telling. :P
(Score: 0, Disagree) by Anonymous Coward on Thursday April 16 2015, @01:43AM
The answer is June 18.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @01:45AM
No it isn't.
(Score: 3, Funny) by Anonymous Coward on Thursday April 16 2015, @01:47AM
No, it is June 18. I'm Cheryl, and it's my birthday. So you can fuck right off.
(Score: 0, Disagree) by Anonymous Coward on Thursday April 16 2015, @02:05AM
May 19
(Score: 5, Funny) by PartTimeZombie on Thursday April 16 2015, @02:07AM
No, I'm Cheryl, and so's my wife.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:51AM
Dad? Is that you??
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:54AM
Yes it is.
(Score: 3, Informative) by Immerman on Thursday April 16 2015, @01:59AM
Definitely not. Albert's first comment eliminates two months, Bernard's two more days, and then Albert's final comment eliminates two of the three remaining days.
(Score: 3, Informative) by Anonymous Coward on Thursday April 16 2015, @01:52AM
July 16
(Score: 5, Funny) by slinches on Thursday April 16 2015, @02:16AM
I think that's right, but the problem has a flaw. How do Albert and Bernard know what Cheryl whispered in the other's ear? It could have been anything since she didn't explicitly state that aloud. Also, the simpler solution would have been for Albert and Bernard to just tell each other what Cheryl told them.
(Score: 5, Informative) by Anonymous Coward on Thursday April 16 2015, @02:20AM
Cheryl: "I'll tell Albert the month, and I'll tell Bernard the day and if you can figure it out without cheating, both of you get to fuck me."
(Score: 1, Informative) by Anonymous Coward on Thursday April 16 2015, @02:32AM
Little do Albert and Bernard know that Cheryl is actually a pre-op tranny who still posses a rather hairy cock and scrotum.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:41AM
"Albert and Bernard just met Cheryl in a gay bar."
They know about the scrotum.
(Score: 2) by jmorris on Thursday April 16 2015, @03:02AM
Yea, that is the fatal flaw. Each knows either the month or day and from the wording of the problem they are left to ASSume she told the other party the other clue and ASSuming she didn't give either a clue that would instantly give it away. So anyone who answers this one fails because they ASSumed more information that was stated. The only correct answer must be 'unknown due to insufficient data'
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:09AM
Regardless of how the question is stated, the guy who cheats will get the job.
(Score: 1) by khallow on Thursday April 16 2015, @03:29AM
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:46AM
Unless he's too naive to know to cheat.
(Score: 2, Insightful) by khallow on Thursday April 16 2015, @12:27PM
Unless he's too naive to know to cheat.
Naivety is a less permanent condition than stupidity.
(Score: 1, Interesting) by Anonymous Coward on Thursday April 16 2015, @03:37AM
I agree with this. Years and years of "never assume, verify" pounded into my head has made me terrible at riddles but magnificent at getting actual shit done.
(Score: 2, Interesting) by Anonymous Coward on Thursday April 16 2015, @03:36AM
It's a premise of the format that all of the statements are taken as true.
However, even though I'm good at this sort of thing and would like to say, that being good at these things makes you smart, really this is just a game based on a particular form of reasoning. You have a precise logical system where all of the inputs and outputs follow strict rules. This problem is actually similar to sudoku.
For most real life logic, for heavily contrived statements like those seen in this problem, it's actually reasonable to assume that the statement is likely to be faulty (or only partially true) if it's difficult to relate to other statements. Resistance to this form of game is in its way a form of lateral reasoning.
(Score: 2) by DeathMonkey on Thursday April 16 2015, @06:12PM
You have a precise logical system where all of the inputs and outputs follow strict rules.
What a horrible thing to be teaching in a Math class!
(Score: 2) by mr_mischief on Thursday April 16 2015, @04:16PM
SPOILER
She told Bernard the day. There's only one day on there that's unique. Albert knows that if Bernard knows for sure the month that it can only be the day that is unique.
The unique day of the month is 18.
The 18th is only listed for June.
The only answer is June 18th.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @04:45PM
Come on, you're embarrassing.
There are 10 dates with 4 unique months, 2 unique numbers (18,19) and 4 double numbers (14-17).
If A can (somehow) know that B (who knows the day) doesn't know the birthday, then it can't be one of the unique numbers. So May 19 and June 18 are the first to definitely not be the birthday.
I really dislike the first statement by A, because he can't "know that B doesn't know" because B didn't have a chance to speak. So the statement is irrational at this point in time... but it's part of the puzzle that it's true, and part of the "intelligence test" to recognise that. (otherwise no solution exists)
Try again. At most 4 iterations of assumptions are required to solve the problem.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @06:51PM
He DOES know that B doesn't know. He can prove it from the information he was given; the latter fact is part of the information you must use to solve the riddle.
(Score: 2) by mr_mischief on Thursday April 16 2015, @10:33PM
Well, now, you're right that I missed the 19 being a unique number.
The method still works. Albert and Bernard know the birthday but we don't. It's either May 19th or June 18th, but if Bernard was given either the 18th or the 19th, since the 19th is only in May and the 18th in June (and A knows the month) they now both know both the month and day.
(Score: 5, Informative) by TheB on Thursday April 16 2015, @02:16AM
Correct.
*** SPOILERS ***
"Albert: I don’t know when your birthday is, but I know Bernard doesn’t know, either."
Since there is only one 18th and 19th.
This eliminates May and June from the possible months.
This leaves Bernard with
July 14 — July 16
August 14 — August 15 — August 17
"Bernard: I didn’t know originally, but now I do"
If Bernard was told 14th he wouldn't know which month.
so Albert knows that it is either
July 16, August 15, or August 17
"Albert: Well, now I know, too!"
If Albert was told August he wouldn't know which day.
This leaves July 16 as the only possible day.
(Score: 5, Funny) by b on Thursday April 16 2015, @04:06AM
Nice answer! The last step foiled me. The riddle also reminds me of this "joke".
Three logicians walk into a bar. The bartender asks "does everyone want beer?" The first says "I don't know", the second says "I don't know" and the third answers "Yes".
(Score: 1, Informative) by Anonymous Coward on Thursday April 16 2015, @07:09AM
Because "everyone" is a logical AND.
If the first one didn't want a beer, he would know the answer. No, not everyone wants a beer. Even if two of them would want a beer, that's not everyone.
So, when he answers "I don't know", the other two can deduce that he wants a beer.
The same thing goes for the second person.
As both said "I don't know", the third person only needs to answer himself.
(Score: 5, Insightful) by KilroySmith on Thursday April 16 2015, @04:32AM
Your logic is lacking.
Albert doesn't know anything other than what Cheryl told him ("To Bernard, she whispered the day, and only the day"). As far as Albert is concerned, Cheryl could have told Bernard the exact month, day, hour, and second. When he says "I don’t know when your birthday is, but I know Bernard doesn’t know, either", he's making a statement that has no basis in logic, and as a result gives no information to Bernard.
Albert doesn't know that Cheryl told Bernard the day. Any conclusions you draw can't assume that knowledge.
(Score: 1, Insightful) by Anonymous Coward on Thursday April 16 2015, @05:53AM
Yes, yes, I thought of that objection too, but it's also easy figure out what the people who designed the question wanted you to answer.
(Score: 3, Interesting) by TheB on Thursday April 16 2015, @07:11AM
You are right that without an assumption this puzzle is unsolvable.
It is a common error in puzzles and tests. I've seen similar "must read between the lines" questions on college entrance exams. According to one instructor, it was an intentional omission to test ability to make reasonable conclusions of intent.
(Score: 2, Funny) by Anonymous Coward on Thursday April 16 2015, @08:02AM
Well, here's what actually happened:
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @08:04AM
Err ... s/Alice/Cheryl/ of course …
(Score: 1, Funny) by Anonymous Coward on Thursday April 16 2015, @08:25AM
More like they looked it up on facebook etc even if cheryl doesn't list it on facebook, sometimes you can tell from the birthday greetings in her timeline ;).
(Score: 2) by wonkey_monkey on Thursday April 16 2015, @09:49AM
Any or all them could also have been lying at any point, since that's not explicitly stated.
Cheryl's birthday could be August 15, May 16, or January 4.
She might even be a fictional character, and this entire thing is just a tissue of lies, in which case she doesn't even have a birthday.
systemd is Roko's Basilisk
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @04:35PM
Technically you are correct. Also note https://xkcd.com/1475/ [xkcd.com]
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @08:40PM
Incorrect.
Albert knows the month.
Albert knows that Bernard knows the date.
Albert makes the statement "I don’t know when your birthday is, but I know Bernard doesn’t know, either" and passes critical information to Bernard.
By saying that "Bernard doesn't know, either", Albert affirms that for the month that was spoken by Cheryl, all the days in the month are replicated in other months.
Let's take the answer as an example. July has two dates: 14th and 16th. The 14th is replicated in August, and the 16th is replicated in May. Therefore Albert knows that Bernard has no idea what Month the birthday is just by Bernard using his private information (16th).
This is also true of August as all the days (14th, 15th, 17th) are repeated in other months.
Albert could not have made the absolute statement "Bernard doesn't know, either" if the month was either May or June. Both of these months contain a unique day, thus Bernard could have known the birthday using just his private information if he was given 18th or 19th as the day.
If May or June was the correct month, Albert would have said "Bernard could know the birthday" and a different logic path would follow.
(Score: 2) by EQ on Thursday April 16 2015, @09:46PM
What most folks miss is this : By saying that "Bernard doesn't know, either", Albert affirms that for the month that was spoken by Cheryl, all the days in the month are replicated in other months.
It doesn't necessary follow - its a matter of semantics to get that much inference (affirms) out of such a simple statement. The broadest meaning does NOT include that information, and requires a contextual jump that may not be justified in normal conversation. "doesn't know" could be stating the simple fact that he does not know the birthday meaning the month AND day -- which is an allowable and perfectly lgical semantic interpretation of the statement. In that case you cannot draw the inference which the problem assumes that you do. Once you get past this, and make the non-colloquial semantic change in the processing of the statements, the problem is easy. Its not making the logic that's tough, its the contextual jump. For many, this isnt a logic problem, its a trick of semantics problem.
(Score: 4, Interesting) by bzipitidoo on Thursday April 16 2015, @04:35AM
I don't follow it. I guess we have to assume several things. First, Albert and Bernard communicate with each other in some fashion. They don't pool their knowledge, because if they did they'd know immediately. Maybe Albert only sees that Bernard is still stumped? Or knows that Cheryl would not give Bernard a clue that gives away the information? How does Albert know Bernard was told the day? Cheryl could have told them both the same information, the month. Or she could have given Bernard a formula, something like "day = month + 9" or "day = 23 - month".
But, if everyone knows that Cheryl told Bernard which day, we can eliminate May 19 and June 18, otherwise Bernard would know. June 17 can also be eliminated, because if Albert knew the month was June, and knew that Bernard did not know Cheryl's birthday, then Albert would know that it has to be June 17. But I don't yet see why May 15 and 16 are out.
(Score: 3, Insightful) by Anonymous Coward on Thursday April 16 2015, @04:51AM
The first assumption is that Cheryl was even born. How does Albert know that Cheryl was not from her mother's womb untimely ripped?
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @07:42AM
From the puzzle:
“My birthday is one of these,” she said.
This already implies that Cheryl has a birthday, because if she didn't have a birthday, none of the given dates would be her birthday.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @08:18AM
Albert assumes Cheryl has a birthday, before she confirms that she has a birthday.
(Score: 2) by Anal Pumpernickel on Thursday April 16 2015, @02:23PM
What if she was lying?
(Score: 3, Funny) by fritsd on Thursday April 16 2015, @08:30AM
Singapore probably doesn't have room for the Forest of Dunsinane.
(Score: 2) by TheB on Thursday April 16 2015, @07:42AM
Without assuming that both Albert and Bernard are aware of Cheryl whispering the month to one and the day to the other this puzzle is unsolvable.
Given that the reader is given this information it is reasonable to assume that both Albert and Bernard also know this.
May 15th and 16th are out for the same reasoning you used to eliminate June 17th.
If Cheryl's birthday was May 19th then Albert's statement "I know Bernard doesn’t know, either." would be invalid.
Since there is only one 19th Bernard could deduce it must be May 19th. Albert must know the month is not May for his statement to be correct.
(Score: 4, Insightful) by bzipitidoo on Thursday April 16 2015, @02:16PM
Oh, I see it now. Albert knows that Bernard does not know, not because Bernard or Cheryl told Albert that, but because Albert was told a month for which Cheryl did not give unique days. I was thinking that Albert saw only that Bernard did not know, which eliminates only June. So all of May and June are out. After Albert announces that he knows Bernard does not know, Bernard can also make that connection.
Then, the 14th is out, because Bernard would still not know which month Albert was told if the day was the 14th, and he says he does know after hearing Albert. We're down to July 16, Aug 15, and Aug 17.
Finally, August is eliminated because Albert says that because Bernard now knows, he knows too. If Albert knew the month was August, he still could not tell if it was the 15th or 17th. So Albert must have been told that the month was July, for him to be able to announce (truthfully) that now he knows too.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @07:48AM
Yes, you have to assume they both know that Bernard was told the day and Albert was told the month. The only other communication between them is in what they explicitly said in this problem.
Now if Albert can say he knows Bernard doesn't know when Cheryl was born, then as Both May and June have unique days in it, Albert knows it is neither May nor June, that lets Bernard know it isn't May or June, now Bernard says he does know with this new information, which leaves only one option.
(Score: 2, Insightful) by hoeferbe on Thursday April 16 2015, @04:50AM
OK, I understand how this eliminates June as a possible month from Albert's point of view, but I don't understand how it removes May.
Taking away 18 as a candidate would leave June 17th as the only June choice. If Albert had been told "June" by Cheryl, then Albert would now know her birthday is June 17th. But since Albert doesn't know that, it removes June as a candidate.
In Albert's mind, these should still be the possibilities:
May 15 16
Jul 14 16
Aug 14 15 17
What is the reasoning that Albert can eliminate the entire month of May before Bernard says "I didn’t know originally, but now I do"?
(Score: 4, Informative) by KilroySmith on Thursday April 16 2015, @05:48AM
Well, if you ignore my comment above and assume that Bernard knows that Albert knows the month, and Albert knows that Bernard knows the day, then...
Following TheB's analysis,
"Albert: I don’t know when your birthday is, but I know Bernard doesn’t know, either"
If Cheryl had told Bernard a unique date (i.e. the 18th or 19th which only occur in one month), then Bernard would immediately know both the month and date. With this statement, Albert is telling Bernard that the Month that Albert was told doesn't have a unique possible birthday date - that for the month he was told, each of the possible dates also occur in a second month. That tells Bernard that Albert must have been told either July or August, because both May and June have unique dates (the 19th of May or the 18th of June).
I think you're getting a bit confused at this step. Albert couldn't make his statement "I know Bernard doesn't know either" if it's possible that Bernard was given either the 18th or 19th. Albert knows, based on the month he was given, that Bernard couldn't possibly have been given a unique date, so Albert must have been given a month that doesn't have a unique date.
Bernard now knows two pieces of information - a date that occurs in at least two different months, and the fact that May and June have been eliminated. The 14th occurs in both July and August, but can't be the right date - if it was, Bernard wouldn't be able to make the statement that "I didn’t know originally, but now I do". If the date was the 14th, the information that Albert has given him wouldn't be sufficient to choose one or the other. So, we can eliminate the 14th.
July 16 is a possibility. The 16th occurs in both May and July, so Bernard wouldn't be able to tell the difference originally, but would be able to by using Albert's revelation to eliminate May 16.
August 15 is a possibility. The 15th occurs in both August and May, and Bernard should be able to eliminate May using Albert's revelation.
August 17 is also a possibility. The 17 occurs in both August and June, and Bernard should be able to eliminate June using Albert's revelation.
So how do we choose between these three possibilities?
Bernard reveals the next clue - "Well, now I know too". So, of the three possibilities, only one can be possible
If Albert had been told August, then he couldn't determine whether Aug 15 or Aug 17 was the correct one, so it can't be either of those or he couldn't make the statement.
If Albert had been told July, then he could make the statement. This is the only possibility left, so it must be the correct date.
(Score: 4, Informative) by Ryuugami on Thursday April 16 2015, @05:50AM
If B was told "18th", he would know it was May 18th.
If A was told "May", there would still be a possibility of the day being May 18th, so he wouldn't know B has insufficient information.
In other words, as the first step you can strike all months that have any unique days.
If a shit storm's on the horizon, it's good to know far enough ahead you can at least bring along an umbrella. - D.Weber
(Score: 2) by fritsd on Thursday April 16 2015, @08:32AM
Cheryl also says aloud to both Albert and Bernard: "I have told Albert the month and I have told Bernard the day"
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @08:48AM
"Albert: I don’t know when your birthday is, but I know Bernard doesn’t know, either."
How does A know B doesn't know? C hasn't even asked B anything and thus B hasn't even said anything yet!
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @01:11PM
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:21AM
It took me a good ten minutes to sort it out. Maybe I'm not all that much stupider than Singapore kids.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @02:30AM
Did you know there are more people with genius IQs living in China than there are people of any kind living in the United States?
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:33AM
Did you know the number of people in China would have to increase by 3000% or for the population of the United States to decrease by 96.6% for that statement to be true?
Alternatively adjust the IQ requirement for genius to be only 108.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:52AM
It's a quote from The Social Network (2010) and the very next line is, "That can't possibly be true."
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:08PM
the number of people in China would have to increase by 3000%
What sort of fucked statement is that?
If the population decreases by a measly 100%, then there is no more population.
Please, what number are you hoping I'd find after your extra 2900% of nothing?
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:19PM
Moron! Learn to read betta!
Dagnabit, and I even quoted the "increase" - and still failed to read it!
Apologies for the knee-jerk, self-righteous, assholistic(-but-about-something-else) post-in-haste.
(Score: 1, Interesting) by Anonymous Coward on Thursday April 16 2015, @03:07AM
I don't know much about Singapore but I don't think Albert, Bernard, or Cheryl are common names in the country.
Anyone have any insight on why these names were used or who put together the questions?
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @03:12AM
They're phonetic spelling names. Albert-Bernard-Cheryl, A-B-C.
(Score: 1, Informative) by Anonymous Coward on Thursday April 16 2015, @03:21AM
No, English first names are quite common there, like in Hong Kong.
(Score: 3, Insightful) by zugedneb on Thursday April 16 2015, @04:03AM
Of course, practice makes the master, but there are some problems that seems to "alter" the way the brain works, and to me (expert on all kinds of things), this altering seems more common when dealing with combinatorial/logic problems, than with analysis problems.
As example, if you work a lot with decomposing problems into state machines, you get really good at it, after a while. And then there is logic.
Being good at this kind of problems is not as much intelligence as rewiring in the brain.
I remember, in a weekly paper published by the an engineering union, they had a look microsofts habit of asking puzzles in interviews. They concluded that if interrogator does not like the applicant, but needs some reason to dismiss the person, a failed puzzle solving was always a good reason.
old saying: "a troll is a window into the soul of humanity" + also: https://en.wikipedia.org/wiki/Operation_Ajax
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @04:24AM
When the unsuccessful applicant can solve every puzzle and answer every technical question correctly, the interviewer gets desperate and proceeds to trivia questions like "Who are the K and R in K&R C?" and social questions like "Which of K or R have you met personally?"
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @06:14AM
And when those fail they go for the ink blotches. Can't win at a personality test.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @04:25AM
Shut the fuck up. "not as much intelligence as rewiring in the brain" what the fuck does that even mean? what is "intelligence" and how is it different from "rewiring in the brain"?
(Score: 1) by zugedneb on Thursday April 16 2015, @05:22AM
Well, you can train yourself to hear music, as example.
Although I am not a composer, I trained myself to hear trance or classical baroque music, mostly violin and cembalo.
This is a form of "rewiring" - you can actually hear your imagination.
The "rewiring in the brain", also come from my experience with electronics and state machines - it is difficult and slow at first, but slowly and with practice, problems turns "transparent", it is easy to see the state machines in them. It is almost as having the technique "imprinted" in the mind...
Of course I might be wrong, and this is just a part of mind that handles language.
old saying: "a troll is a window into the soul of humanity" + also: https://en.wikipedia.org/wiki/Operation_Ajax
(Score: 1) by zugedneb on Thursday April 16 2015, @05:31AM
Even better example, old people "say", and I believe them, that if you make calculations a long time with abacus, you will develop an internal abacus, and kind of "see" it with the minds eye, and be able to make calculations as fast as with a real abacus.
old saying: "a troll is a window into the soul of humanity" + also: https://en.wikipedia.org/wiki/Operation_Ajax
(Score: 2) by hubie on Thursday April 16 2015, @11:44AM
I would guess that it only applies to a certain subset of people. Feynman address this [caltech.edu] within the context of counting the passage of time. Feynman found that he could accurately count the passage of time in his head while doing various things, particularly reading, but he couldn't when he was talking. He mentioned this to John Tukey and Tukey couldn't believe Feynman could do it while reading because Tukey could do it while reading but not talking. It turns out that Feynman "heard" his counter ticking in his head while Tukey "saw" the numbers go by in his head:
(Score: 2) by Geotti on Thursday April 16 2015, @12:24PM
So, who of the two could count while talking and who while reading?
(Score: 2) by hubie on Thursday April 16 2015, @02:27PM
As I recall, Feynman could count while reading but not talking. Tukey the other way. Tukey apparently "saw" a ticker-tape with numbers scrolling by in his mind.
(Score: 2) by Geotti on Friday April 17 2015, @12:20AM
Hmm, I wonder how the the other learning types like olfactory and kinesthetic/somatic are counting (if I assume correctly that Feynman is an auditory and Tukey a visual type)...
(Score: 4, Funny) by meisterister on Thursday April 16 2015, @04:42AM
It's obvious that Albert is lying and Bernard murdered the suspect in the Library with the candle holder!
(May or may not have been) Posted from my K6-2, Athlon XP, or Pentium I/II/III.
(Score: 1, Funny) by Anonymous Coward on Thursday April 16 2015, @05:09AM
Murdered the suspect? Is this some kind of Internal Affairs investigation?
(Score: 2, Interesting) by Anonymous Coward on Thursday April 16 2015, @05:12AM
I will rephrase the dialog.
The original phrasing was unnecessarily difficult, la.
Albert (knows month) says to Bernard:"Dude, I don't have enough information to know when her birthday is, but looking at the possible dates that she gave us I can tell you that I can be sure that you cannot know either *wink*"
Bernard (knows day) gets the hint and says:"Well thanks Albert. With that information you just gave me I can deduce her birthday and now I know it."
Albert also gets Bernards hint and says:"In that case I now also know when her birthday is."
srsly though, I read the original problem/question (horrible la) and had to lookup the solution.
the original phrasing/story was terrible and i wonder if as a side effect maybe kids that can solve problems like this might grow up learning to make things more difficult then need be (for others).
I still believe that language is here to convey ideas and information and that abusing it "to make things more difficult" is counterproductive (to using language).
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @08:31AM
agreed.
and there you are, mixing up "then" and "than" ... go figure!!
(Score: 3, Interesting) by bradley13 on Thursday April 16 2015, @06:13AM
For what it's worth, the results seem to come from this table in the 2012 PISA study [oecd.org]. It's the usual sort of ranking that we see over and over:
1. Korea, Singapore, China, etc.
2. Western Europe plus US/Canada/Australia
3. Eastern Europe, Russia
4. Everyone else
There are some other interesting tidbits in the graph. For example, with few exceptions, boys significantly outperform girls. This is true in all of the top-performing countries, and in most of the rest (with a couple of interesting exceptions).
Everyone is somebody else's weirdo.
(Score: 3, Funny) by wonkey_monkey on Thursday April 16 2015, @07:21AM
I don't see any angry arguments yet. Better do the Monty Hall problem next.
systemd is Roko's Basilisk
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @07:54AM
Then you have not been looking very hard. [soylentnews.org]
(Score: 2) by wonkey_monkey on Thursday April 16 2015, @10:03AM
That's not nearly angry enough for my liking.
systemd is Roko's Basilisk
(Score: 3, Informative) by WillR on Thursday April 16 2015, @01:16PM
(Score: 2) by theluggage on Thursday April 16 2015, @01:51PM
I don't see any angry arguments yet.
It's hard to find much to get angry about here. OK, so, technically, the question does not recount that Cheryl said aloud that she was telling Albert just the month and Bernard just the day - but nothing that Albert or Bernard say subsequently makes any sense otherwise, so its a perfectly sensible assumption that leads to a consistent answer. If you want questions to be 100% logically watertight, present them in mathematical notation, don't tell a little story.
It probably helps that your typical Soylent reader shouldn't really have any problem with jotting down a 2-way table and eliminating rows and columns.
I think Monty Hall gets more fun because (a) probability is harder than this sort of simple logic and its more likely that someone who should know better will screw up the math and (b) it effectively opens the can of worms of what probability means in the case of a 1-off event. Personally, I found that any mystery about the answer vanished when I sat down to write a program to simulate 1000 games. Its when you try and get into the head of the contestant deciding whether to stick or switch that it turns into Schroedinger's Goat (Seriously, you're only playing once, how will you ever know that the odds 2/3 and not 1/2 or 1/3? Even Monty's goat feed protection* is going to depend more on how persuasive he is than the theory). Again, you can get bogged down in deconstructing the question if you like, but I'll bet you 50 internets that the questioner was looking for "2/3".
Also, it depends on how you feel about goats [xkcd.com].
(*if Monty is paying to advertise Chevrolet on prime-time TV then he needs a new manager, but I'm sure the sponsors don't pay the vet's bills for the goats).
(Score: 2) by PizzaRollPlinkett on Thursday April 16 2015, @11:08AM
This looks like one of Raymond Smullyan's Knight and Knave puzzles from his plenteous number of books (check out Dover's web site) about logic puzzles. Even the most advanced logician has to build truth tables to get through these puzzlers, which often don't contain enough information to actually solve the puzzle, but give you enough information to use the process of elimination. This Singapore puzzle looks like one of those. You can't answer it, but you can use the process of elimination to narrow down the choices. Probably why it has people stumped. If you know how these Knight and Knave problems are structured, it makes it a lot easier to approach something like this because they're all basically the same. If you're a Singapore teenager who has never seen one, it would be a ... challenge.
Smullyan doesn't quit, either. His new "A Beginner's Guide To Mathematical Logic" is excellent. Just came out last year. Highly recommended!
(E-mail me if you want a pizza roll!)
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @01:16PM
Now write the problem in prolog.
(Score: 2) by RamiK on Thursday April 16 2015, @02:16PM
It's deductive reasoning. Specifically, it's a 3 round elimination problem in a two dimensional set.
It's the kind of nonsense you pick over a weekend or two before or during introduction to algorithms or even combinatorics.
Definitely not something a 10 year old should waste time specializing on.
compiling...
(Score: 1, Insightful) by Anonymous Coward on Thursday April 16 2015, @05:15PM
"This isn't a math problem, it's a math problem."
Part of the problem with the American educational system is a bizarrely narrow view of what is and isn't "math".
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @09:05PM
I agree. There is no equal sign so it;s not a Math problem to 90% of Americans.
(Score: 2, Informative) by purpleland on Friday April 17 2015, @01:14AM
During my university years I met many Singaporean scholars there sponsored by their government.
They pretty much kicked everyone's butt in all the mathematical exams. But it was interesting to observe that even though most everyone in the course were straight A students, only about half of us had a knack for programming and there didn't seem to be much correlation between math ability and ability to program, though being good at math definitely makes a good programmer truly awesome.
Being government scholars they were all required to work for government for at least a few years after graduation. And many seem to remain in the public sector for many years. I get the impression their government works pretty well for the most part.
I also lived in Singapore for several years as an expatriate. Failing companies in the private sector that are somehow regarded as important are often acquired by the government and turned around.
There is something to be admired about a society that is so focused on math and education, and I think has enabled that little country to thrive way ahead of its neighbors. Personally, I'm relieved not to be in their meritocratic rat race.