A professor tells his class that he will give them a surprise examination sometime next week.

For our purposes, an examination is considered a "surprise" if on that day, the class is not 100% sure that they will get an examination. (If they only suspect it, the examination can still be a surprise.)

Having given the class this warning, is it still possible for the professor to give them a surprise examination next week? (Assume that he will not give them an exam if it will not be a surprise.)

"For our purposes, an exam is considered a surprise if on that day, the class is not 100% sure..."

The solution's first step is to rule out Friday, but you can only rule it out if "the class" is 100% sure. YOU might conclude that it's not Friday, but can you assume that the entire class has logically concluded that it can't be Friday? If even only one student does not conclude this, then "the class" is not at 100%, and Friday is fair game.

Posted by Greg on 2003-01-07 22:08:54