Ok, I am sorta stuck on this, and was hoping someone could nudge me in the right direction. The question is: An instuctor gives her class a set of 10 problems with the information that the final exam will consist of a random selection of 5 of them. If a student has figured out how to do 7, what is the probability that he or she will answer correctly a) all 5 problems. b)at least 4 of the problems. This seemed so simple. To start, I took 10 choose 5 as the possible test question combinations, since they were to be randomly picked. Then I took 7 choose 5, for the 5 answers the student would know. Then I divided 7 choose 5 by 10 choose 5. This worked out right, and answered question a) correctly. I tried to apply this same line of thinking to b), taking 7 choose 4 and dividing it by 10 choose 5, but my answer is way off according to the book. So, did I just get lucky on the first one, or am I on the right line of thought?