There are 2 doors in a room. Behind one of them there is hell and behind the other one is heaven. There is a person in front of each of the doors. They know where heaven is and where hell is. One of them always lies and the other one always says the truth. Can you choose the door to heaven if you are only allowed to ask one question?

Answer:

Yes. You have to ask one of the persons, if the other person would say that heaven is behind door 1.

Proof:

Lets assume door 1 is heaven. If you ask the liar he would answer no since the other person would say yes to the question if door 1 is heaven. If you ask the person that says the truth, he would also say no since the other person would say no to the question if door 1 is heaven. Lets assume door 1 is hell. If you ask the liar he would answer yes since the other person would say no to the question if door 1 is heaven. If you ask the person that says the truth, he would also say yes since the other person would say yes to the question if door 1 is heaven. To conclude, door one is heaven if the answer to the question is no otherwise it is hell.

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