**Factorial**

A number*factorial*means to multiply that number by all the whole numbers below it. It is written with a exclamation (!) mark after it, like this: 5!, and means:

5! = 5 x 4 x 3 x 2 x 1 = 120

6! = 6 x 5 x 4 x 3 x 2 x 1 = 720

Now here is a question for you:

What is 4!?