Sarah wants to order a pizza with 2 different toppings. She has 9 different toppings to choose from. How many different pizzas could she order?
In her first topping, she has 9 choices.
In her second topping, she has 8 choices. (not 9 because it can't be the first one she chose)
9*8 = 72 options.
However, we have to consider that choosing a then b is the same as choosing b then a.
For example, choosing peppers then sausage is the same as sausage then peppers, so we have to divide by 2.
72/2 = 36.
36 options. :))