An instructor gives an exam with thirteen questions. Students are allowed to choose any ten to answer. (a) How many different choices of ten questions are there?
(b) Suppose five questions require proof and eight do not.
(1) How many groups of ten questions contain three that require proof and seven that do not?
(ID) How many groups of ten questions contain at least one that requires proof?
(HI) How many groups of ten questions contain at most three that require proof?
(c) Suppose the exam instructions specify that at most one of questions 1 and 2 may be included among the ten. How many
different choices of ten questions are there?
I
(d) Suppose the exam instructions specify that either both questions 1 and 2 are to be included among the ten or neither is
to be included. How many different choices of ten questions are there?