probability
posted by Sharon .
A pizza shop offers nin toppings. No topping is used more than once. What is the probability that the toppings are pepperoni, onions, olives, and mushrooms? Round your answer to the nearest thousandth.

Excel spreadsheets are very helpful for these kinds of problems.
There are 9choose0 ways =1 to have a pizza with zero toppings.
9choose1 ways = 9 to have 1 topping
9choose2 = 36 ways to have 2 toppings
9choose3 = 84 ways to have 3 toppings
Repeat for all 4 through 9 toppings.
Then sum all your answers. This is the denominator. The numerator is one.