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Permutations

There's actually a formula for this -- and a button on your calculator too!

The number of ways to arrange (permute) n objects taken r at a time is

n! / ( n - r )!


Let's check this with our last two problems:

8 books taken 3 at a time...

n = 8 

r = 3

 

n! / ( n - r )! = 8! / ( 8 - 3 )! = 8! / 5! = 8 * 7 * 6 * 5! / 5! = 8 * 7 * 6 = 336

 

YOUR TURN:

10 books taken 4 at a time...