The Multiplication Principle!

If an action can be performed in X1 ways, and for each of these ways another action can be performed in X2 ways, then the two actions can be performed together in X1X2 ways. This is only if you don't have restrictions.

Restriction:A set of rules of what you cant use or do to find your set of how many ways.

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Ex.1: There are 7 people in line at a restaurant; how many different ways can their order be taken?

You would multiply 7*6*5*4*3*2*1, and the answer would be how many ways.

This can also be expressed as 7!.

7!=5040

Ex.2: If Jimmy has 3 different shirts and 5 different pants, how many combinations does he have?

You would multiply 3*5 and that would be your answer.

Jimmy has 15 different shirt/pants combinations.