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?

My first insights are these.
I will call the day mentioned in 2 Day A,the day in 3 Day B,and the day in 4 Day C.
First,Day A and Day C cannot both be truth-telling days,because there is only one truth-telling day.
Therefore,Bob's truth-telling day is either Monday,Tuesday,Wednesday,or Friday.
We call this fact the important fact.
Day B cannot be the truth-telling day,because if it is,then the truth-telling day would be either Thursday,Saturday,or Sunday,contradictory to the important fact.
Therefore,Day B is a lying day,so Day B is either Monday,Tuesday,Wednesday,or Friday.

Posted by Tim Axoy on 2003-03-22 14:50:24