If A pizza restaurant offers 9 choices of vegetarian toppings for pizza In how many ways can you choose 4 toppings

You have to pay attention in this:

- The order doesn't matter

- No repetition

- You can not chose all available elements (just 4)

So in this case your formula must be: 9C4 = 9! / (4! * (9-4) !) = 126 ways