We have 8 possible letters, and we are forming (ordered) 5-letter sets. So the number of permutations is given by:

n!/(n-r)! with n=8 and r=5. This is for selecting permutations *without replacement*, i.e. we can only use each letter once in each word, aaaaa would not be allowed for example.

This is:

8!/(8-5)!

= 8!/3!

= 8x7x6x5x4

= 6720

Mar 21st, 2015

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