Rule of product
Examples

In this example, the rule says: multiply 3 by 2, getting 6.

The sets { A, B, C } and { X, Y } in this example are disjoint sets, but that is not necessary. The number of ways to choose a member of { A, B, C }, and then to do so again, in effect choosing an ordered pair each of whose components are in { A, B, C }, is 3 × 3 = 9.

As another example, when you decide to order pizza, you must first choose the type of crust: thin or deep dish (2 choices). Next, you choose one topping: cheese, pepperoni, or sausage (3 choices).

Using the rule of product, you know that there are 2 × 3 = 6 possible combinations of ordering a pizza.
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