You have pepperoni, mushrooms, onions, and green peppers. How many different pizzas can you make by using one, two, three, or four of the toppings?

Answer 1

16 if you count a pie with no toppings, otherwise 15.

Count 'em up.

4 with one topping

6 with 2 toppings

4 with 3 toppings

and 1 with 4 toppings.