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I just happened to see this story appear in our #rss-bot feed. How to Solve the World's Hardest Logic Puzzle. Given that this is the weekend, I thought it might make for an interesting challenge and discussion.
To set the stage for the puzzle, the author provides some background on Raymond Smullyan, the puzzle's composer:
While a doctoral student at Princeton University in 1957, studying under a founder of theoretical computer science, Raymond Smullyan would occasionally visit New York City. On one of these visits, he met a "very charming lady musician" and, on their first date, Smullyan, an incorrigible flirt, proceeded very logically—and sneakily.
"Would you please do me a favor?" he asked her. "I am to make a statement. If the statement is true, would you give me your autograph?"
Content to play along, she replied, "I don't see why not."
"If the statement is false," he went on, "you don't give me your autograph."
"Alright ..."
His statement was: "You'll give me neither your autograph nor a kiss."
It takes a moment, but the cleverness of Smullyan's ploy eventually becomes clear.
A truthful statement gets him her autograph, as they agreed. But Smullyan's statement, supposing it's true, leads to contradiction: It rules out giving an autograph. That makes Smullyan's statement false. And if Smullyan's statement is false, then the charming lady musician will give him either an autograph or a kiss. Now you see the trap: She has already agreed not to reward a false statement with an autograph.
With logic, Smullyan turned a false statement into a kiss. (And into a beautiful romance: The two would eventually marry.)
Clever! But enough with the setup — What's the puzzle?
The Hardest Logic Puzzle Ever goes like this:
Three gods A, B, and C are called, in some order, True, False, and Random. True always speaks truly, False always speaks falsely, but whether Random speaks truly or falsely is a completely random matter. Your task is to determine the identities of A, B, and C by asking three yes-no questions; each question must be put to exactly one god. The gods understand English, but will answer all questions in their own language, in which the words for "yes" and "no" are "da" and "ja," in some order. You do not know which word means which.
The story's author is, himself, a bit of a puzzle-poser. The story tells how to solve the puzzle, but does not actually provide the solution. Are there any Soylentils up to the challenge?
(Score: 3, Interesting) by Jiro on Monday October 24 2016, @05:42AM
"If I asked you X, would you say 'da'?" will produce the answer "da" if the answer to X is "yes" and "ja" if the answer is "no", regardless of whether the person is True or False and regardless of whether "da" means "yes" and "ja" means "no" or vice versa.
If you modify this to "If I asked you X and you responded using the truthteller status you used in answering this question, it will also work on Random. That lets you solve the puzzle in three questions since you have 3 bits of information for 6 possibilities.
You can actually solve it in 2 questions. Since Random always speaks truly or falsely, there are questions which he cannot answer (such as "is your answer to this question a lie?"). Your question would simply be "If I am Random, then (unanswerable question), else (question from the previous paragraph)", appropriately rephrased to be a question.
I'm sure there is an answer that is simpler, but less systematic.