Question says that a yogurt shop allows its customers to add, for no charge, 3 toppings to any yogurt purchased. Also the store has 15 possible toppings.
Now we need to find about how many different 3-topping combinations can a customers choose.
To find that we just need to apply combination formula

which is used to select r items out of n items
we have to find combination of 15 toppings into 3 toppings so we calculate C(15,3) using above formula






Hence final answer is 455.