One of my teachers gives his students essay finals. First, he tells us three numbers, A, B, and C. He gives us A essay questions to study before the test. He picks B essay questions to put on the test, and we must pick out C of them to answer. He tells us to study A-B+C of the given questions if we want to pass the final.

As a procrastinator, I only studied the night before. Luckily, some other students had taken the test a day early, and could tell me which of the questions the teacher had picked. Of course, the teacher would pick a different combination of questions to give to the rest of the students. After hearing which questions were given, I realized I needed to study N less questions than was necessary before!

Find N, generalizing to all possible A, B, and C.

(In reply to Question turns up to be clear by Jonathan Chang)

I find that on the internet, the tone of comments like these are often misread.  I was not at all upset.  Don't worry about it.

I can't say I understand your solution.  A, B, C, and N all represent numbers, not questions.  I'm afraid your answer is incorrect.

Have you read the other comments?  You might find out what you're doing wrong by seeing how other people have looked at the problem. If you want to solve it yourself though, you might want to skip the third comment.  Richard has the answer, though he hasn't proven it.

Posted by Tristan on 2005-04-15 05:07:09