Friday, October 23, 2009

Castle Doors

You find yourself locked in a castle, where the only two doors out are guarded by two men. One of these men always lies and one of them always tells the truth. Unfortunately, you do not know which man is which. One of the doors leads to freedom and one to captivity. Determine a single question that you may ask one of the guards that would reveal the door to freedom with certainty.

This is yet another puzzle from the Fermat's Theorem movie described here.

Submit your answer on our Family Puzzle Marathon site. Solve three puzzles and get a prize!


prluhmann said...

You could ask either of the guards, "Is the liar guarding the door to freedom?" If the liar is guarding the door to freedon, he will answer No. If the truth-teller is guarding it, he will answer No. Therefore, if the guard says No, his door is the one to freedom. If the liar is guarding captivity, he will answer Yes. Similarly, if the truth teller is guarding captivity, he will answer Yes. So if he answers yes, this is the door to captivity, and you pick the other door.

Maria said...

prluhmann, you are very good!
3rd point and a prize. Please email me your mailing address to maria-at-marialando-dot-com, so I can send you a prize.

As this puzzle was a bit tricky and the explanation difficult to grasp in the comments format, I thought of trying to explain prluhmann's answer with a table:
the question that I am referring to is "Is the liar guarding the door to freedom?"
we can see two scenarios: Liar is next to the freedom door, and Truthteller is next to the freedom door - as separate rows of this table.
Who is guarding | Liar's | Truthteller's
the Freedom door | answer | answer
Liar | No | Yes
Truth teller | Yes | No
In either scenario, we see that whoever answers "no" stands next to the door to freedom.

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