Question

A pizza place offers 10 topping choices for their pizzas. How many different two-topping pizzas can you order?
A.
2
B.
45
C.
90
D.
100

Answer 1

The number of ways you can select r-items from a set of n is given by the formula...
\[\Large _nC_r = \frac{n!}{r!(n-r)!}\]
In this case, you have 10 toppings to choose from, so n = 10
and you pick two toppings, so r = 2

Go for it :)