Professor Z was killed by one of his four students, who was helped by another of the four. His students declared:

A: If B is guilty of something, then C must be innocent.
B: If A is innocent, then C must be guilty.
C: If B was the killer, then D must have had nothing to do with the crime.
D: I am innocent.

As everybody should know, guilty parties always lie, and innocent people always tell the truth. Who killed the professor, and who was his accomplice?

If A was guilty, the only way he could be telling a lie would be if B also was guilty, and C wasn't innocent -- and that would imply three guilty parties. A similar reasoning shows that if C cannot be guilty either, so B and D are the guilty ones.

As C told the truth, if B was the killer, D would be innocent; but as we know that D was guilty, then B wasn't the killer -- he was the accomplice, and D thus was the killer.

Posted by e.g. on 2004-03-08 14:00:00