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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: 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.