You have been brought into a room with 2 doors in it.By each door stands a man.You can stay in prison for the rest of your life or ask a question directed to one of these men.One man is standing by the door of freedom and the other is standing by a door behind which is a firing squad.One wants you to die, the other wants you to go free.But you don't know which is which.Further, there is no guarantee that the man who wants you to die is standing by the door of death.Also, one man is bound to always lie and the other is bound to always tell the truth.What single question can you ask 4 freedom?
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I might to point any door out and to ask any guy a question about behind of chosen door, like this:
"What will be an answer of your colleague if I ask him whether is there firing squad/freedom?".
The response is lying at any case, so if you hear "no" it means "yes".
but one of them always tells the truth
OK, let's think about. If you ask the man who always tells truth "What will be an answer of your colleague if I'll ask him whether is freedom behind this door?" what will he answer if there is really freedom? I think, he must just repeat the answer of his deceptive colleague, so he'll say "no". If you ask the same question to lier, he'll flips the answer of truth man who of course would answer "yes", so the answer again will be "no". So you are able to figure out where is freedom door, aren't you?