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.)

It can't be on Friday, it would be known that it's happening then if it's not before. Therefore, it can not be on Thursday, because Friday has been ruled out. This same arguement can be brought back to the beginning of the week. Therefore there can't be any exam at all.

Posted by Half-Mad on 2002-05-12 20:57:58