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?

The riddle can be solved with two questions.  The key is to isolate the liar as quickly as possible.  To do so, first, we must ask Bert if he said that Carl is not a knight.  Two scenarios follow from this question. 

(1) Bert says "Yes", he did say that Carl is not a knight.  If this is the case, then both Bert and Adam have the same story of events and this means that both are telling the truth.  In this case, Carl must be the liar.  We then need a second question to either Bert or Adam (doesn't matter whom we ask because we have established that neither is the liar) to list the identities of all three.

(2) Bert says "No", he never said any such thing in regard to Carl.  In this case, we know that either Adam or Bert is lying, as both cannot be telling the truth.  To determine the identity of everyone, we must  only ask Carl to list each person and his identity (he is not the liar --either Adam or Bert is -- and therefore must tell the truth). 

Posted by Liam on 2005-02-16 20:52:02