Answer
See explanation below
Work Step by Step
a) combinations is a set that can be taken from another regardless of order.
It can be expressed as $C(n,r) = \frac{n!}{(n-r)!r!}$
b) $2^n$ by definition as you can choose whether or not to include an element in the set, and is thus binary.
c) Number of pizzas is simply $2^{10}$
and the number of $3$-topping pizzas is $C(10,3) = 10!/(3!*7!) = 120$
