Chapter 10 - Section 10.6 - Counting Principles, Permutations, and Combinations - Exercise Set - Page 1103: 19

This is a problem involving permutations.

Here, we get that four letter passwords can be formed from the letters A, B, C, D, E, F and G. The order in which the letters are chosen makes a difference because each of the letters (A, B, C, D, E, F and G) is different and no repetition is allowed. Order matters. Thus, this is a problem involving permutations.