Answer
840
Work Step by Step
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