Consider three classes, each
consisting of n students. From this group of 3n students, a group
of 3 students is to be chosen.

a. How many choices are
possible?

b. How many choices are there in
which all 3 students are in the same class?

c. How many choices are there in
which 2 of the 3 students are in the same class and the other
student is in a different class.

d. How many choices are there in which
all 3 students are in different classes?