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.

I was kind of tired, but after looking at the problem a little more, I found a quick solution.

The minimum number of questions is three, because each question must be a yes or no question, and therefore only have two possible answers. The number of possible combinations of results are 8:

Alex=liar/knight; Bert=liar/knight; last speaker=Bert/Alex

Therefore, you must ask at least three questions, because there must be at least 8 possible combinations of answers.

Once proven that three questions are needed, it is a simple matter to find them.

1. What would you say if I asked you if Alex is a knight?
2. What would you say if I asked you if Bert is a knitht?
3. What would you say if I asked you if you were Alex?

Of course there are many possible solutions.

Posted by Tristan on 2003-12-30 19:58:03