Question

A local pizzeria offers 15 toppings for their pizzas and you can choose any 3 of them for one fixed price. How many different types of pizzas can you order with 3 toppings?

Answer 1

Answer:

  455 or 680, depending

Step-by-step explanation:

If we assume the three choices are different, then there are ...

  15C3 = 15·14·13/(3·2·1) = 35·13 = 455

ways to make the pizza.

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If two or three of the topping choices can be the same, then there are an additional ...

  2(15C2) +15C1 = 2·105 +15 = 225

ways to make the pizza, for a total of ...

  455 + 225 = 680

different types of pizza.

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There is a factor of 2 attached to the number of choices of 2 toppings, because you can have double anchovies and tomato, or double tomato and anchovies, for example, when your choice of two toppings is anchovies and tomato.

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nCk = n!/(k!(n-k)!)