You are at one end of a hall blindfolded. At the other end are Alex and Bert. You are allowed to ask any yes or no questions you want, but only one of them will respond. They are standing next to each other and their voices are identical, so you have no way of telling which one responded by sound alone.

Using as few questions as possible, find out the type of each Alex and Bert and who answered your last question.

You only need to ask one question: "If I were to ask you if your name is Alex, would you say yes?"

If Alex answers and is a truthteller, he will say yes. If he is a liar, he will also say "yes", since he would say "no" if you were to ask him if his name is Alex. Similarly if Bert answers and is a truthtelller, he will say "no". If Bert is a liar, he will again say "no", since he would answer "yes" if asked if his name is Alex.
Edited on December 30, 2003, 2:03 pm

Posted by Penny on 2003-12-30 14:01:59