n is the number of things to choose from,
r is how many of them we choose, and
order matters
The exclamation point ! means factorial, i.e., the
product of an integer and all the integers below it.
Example: 6! = 6 • 5 • 4 • 3 • 2 • 1
n! 4! 4 • 3 • 2 • 1 24
———– = ———– = ——–––—– = ——– = 24
(n – r)! (4 – 3)! 1! 1
.
