## Thinking Mathematically (6th Edition)

The number of possible combinations if r items are taken from n items is nCr = $\frac{n!}{r!(n-r)!}$ The number of different ways four-letter passwords can be formed from the letters A, B, C, D, E, F, and G if no repetition of letters is allowed: = 7C4 x 4! =$\frac{7!}{4!(7-4)!}$ x 4! =7*6*5*4 = 840 ways