A class is given a list of 15 study problems from which 10 will be chosen as part of an upcoming exam. A given student knows how to solve 12 of the problems. Find the probability that the student will be able to answer at least 9 of 10 questions on the exam.

The probability that the student will know how to answer the question if all questions were given is

P = 12/15

If only 10 questions are given and at least 9 have to be answered correctly, then the probability would now be:

P = (12/15) (9/10)

P = 18/25 or 0.72 or 72%