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.
You have clarified your question now. A, B & C contain one question each and B question is set from the extraction of A. C question is set from the extraction of A. With the assumption that the teacher would not set the same question as in the past, the student has to master (B+C) less question from the question A (assuming that B & C do not have the identical answers).
For instance, if C has the identical question as some part of question B, the student needs to master B less question.
Every puzzles creators & that includes me might tend to create ambiquous puzzles that bring about more than one answer. An ambiquous answer tends to attract more than one answer. I too make such a mistake in the past & so please do not upset because of my comment. I still enjoy solving your puzzles.
Question turns up to be clear
