Find how many types of pizzas with three toppings can be made.

₈C₃

Find how many pizzas with four toppings can be made.

₈C₄

₈C₃ + ₈C₄ = 56 + 70 = 126

If mushrooms are a known topping, then you choose one fewer topping from only 7 remaining toppings.

$\frac{_{7}C_{3}+_{7}C_{2}}{126}=\frac{56}{126}=\frac{4}{9}$