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A man finds himself in a room with two doors. He has to exit through one of those doors, one door will lead to freedom and the other leads to his death. At each door there is a guard. One of the guards will always lie and the other one will always tell the truth. The man does not know who lies and who tells the truth. He is allowed to ask one of the guards only one question. Assuming that the man wants to survive by exiting through the door that leads to his freedom, what is the question he must ask in order to be certain that he will walk out the correct door?

The man must ask this question: "If i ask the other guard which door will he tell me to open?", and which ever door the guard tells him he should go out the other one. This answer is correct because he does not know if the guard he is talking to is the one that lies or the one that speaks the truth so by asking this question no matter which guard he asks by exiting through the other door he is certain to be free. If the guard he asks is the one that tells the truth he will tell the truth that the other guard would lie and tell him to go out the wrong door so he goes the other way. If the guard he asks is the one that lies then he will lie about the truth that the other guard would say so he goes out the other door again.

Edited by elochai
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Posted · Report post

this is one of my favorites ... it is also the most repeated puzzle on this forum - still coming in new variations (stories) B)) ... long discussion can be found in the thread entitled Honestants and Swindlecants III.

looking forward to new puzzles ... this topic is locked

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