Question 169011
If the letters A, B, C, D, E, and F are used in a five-letter code, how many different codes are possible if repetitions are not permitted? 
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For the 1st letter, you can choose one of 6. For the 2nd, one of the remaining 5, then 4, 3, 2.  So the possibilities are 6*5*4*3*2*1, or 6! (6 factorial).
That's 720.