You are trapped in a dungeon, and see two doors in front of you. One is a way out, andther will lead to certain death.

Two people are standing in front of these doors, one of them a Knight, another - a Liar. Naturally, you do not know who is who, but they know each other's true nature.

Both the people know which way is correct, but you can only ask one question of one of them before making your choice. What will you ask?

Call the people A and B.
Call the doors A and B.
Ask A,"Is B a liar if and only if Door A is the door to escape?"
Suppose A says,"Yes."
If A is a knight,B must be the liar,and A is telling the truth,so A is the door to escape.
If A is a liar,B is a knight,but A is lying,so if and only if B is a knight,A is the good door,so A is good.
Suppose A says,"No."
If A is a knight,then if and only if B is a liar,then B is the good one,but B is a liar,so B is good.
If A is a liar,B is a knight,so really the answer is yes,but only if B is a liar,A is good,so B is good.
If A says,"Yes.",A is good,and if A says,"No.",B is good.

Posted by Tim Axoy on 2003-03-19 07:16:07