The following two facts are known about Adam, Bert, and Carl:

1) Of Adam, Bert, and Carl, there is one knight, one knave, and one liar.
2) Adam said once, "Bert said that Carl isn't a knight."

Can you ask one yes/no question to one person and figure out who is who? If not, how many questions must you ask?

(In reply to re: Two Questions by John)

It would be impossible for both Adam and Bert to be lying, as you claim in your post.  If they both have the same version of events, then both must be telling the truth.

If Bert says "Yes," but he is lying (meaning he never said such a  thing), then Adam could never have said that Bert said that.  Adam would have to tell the truth and saying that Bert said that, when he did not, is a lie. 

If Bert says "Yes," and he is telling the truth, then Adam must also be telling the truth.  Adam cannot be lying in this case because for Adam be lying about what he said, that would mean Bert never said any such thing.  But if Bert confirms that he did say this and he's telling the truth, then Adam must be doing so also.

If it makes more sense to you, you can isolate the liar in other ways.  For example, we could ask Adam straight up whether he ever said that Bert said that Carl is not a knight.  We know for a fact he did say this as it is given to us.  If Adam confirms he said this, then he is telling the truth.  In this case, he cannot be the liar because the liar cannot tell the truth.  If Adam denies saying this, then we know he's the liar and can proceed accordingly.

Posted by Liam on 2005-02-17 13:54:45