Think hard. You probably have the tools.
I like part B. Can you reduce the problem by simply picking one of each of the supplies and then worrying about how to choose 15 with no additional restriction?
A store sells 5 different types of office supplies: pencils, mechanical pencils, pens, erasers, and highlighters. How many ways are there to choose:
a) 20 supplies?
b) 20 supplies with at least one of each office supply?
c) 20 supplies with at least 4 erasers and 2 pens?