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A surprise exam (Posted on 2002-04-26) Difficulty: 3 of 5
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.)

See The Solution Submitted by levik    
Rating: 3.3684 (19 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
re: not true | Comment 15 of 39 |
(In reply to not true by srikumar)

No, you misunderstand the logic. Once Friday has been knocked off, it is permenently out. Simply because, even if we then go on to eliminate Thursday, if the students get to Thursday and they haven't had the exam, it must be on Friday. So our week is now shortened to Monday through Thursday, and we do the same thing all the way up through Monday. However, as other people have said, the puzzle does not end here. Let us suppose that the Students get to Thursday without having the exam. Then they will be certain that the professor cannot give it to them on the Friday. Therefore the professor can give it to them without breaking his word (and could give it on any other day by the same logic).
  Posted by sam on 2003-09-06 15:46:56

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