Question

Erica started her own pizza restaurant. She offers one type of crust, one kind of sauce and 6 different toppings: sausage, pepperoni, mushroom, onions, green peppers and tomatoes.

How many different two-topping pizzas can she make?
Sydney
My guess is 30.
Ms. Sue
Guessing doesn't help, Sydney.

Marth
6 topping choices, 2 toppings per pizza: 6 choose 2, which is 6!/(2!4!)