1. Bob tells the truth on only one day of the week.
2. One day he said: "I lie on Mondays and Tuesdays."
3. The next day he said: "Today is either Thursday, Saturday, or Sunday."
4. The next day he said: "I lie on Wednesdays and Fridays."

On which day of the week does Bob tell the truth?

(In reply to Confusion by Alan)

But if on sunday he said he lied on mondays OR tuesdays, then that would not be a lie, as he does indeed lie on mondays. It is only because he says he lies on both mondays AND tuesdays that telling the truth on tuesday alone makes it a lie. Remember de Morgan's laws: not(A AND B) = (not A) OR (not B) and the similar not(A OR B) = (not A) AND (not B). And the other way of looking at it, the ANDed statement is one statement, not two.

Posted by Charlie on 2003-03-17 17:09:11