For time slot A, he has 4 candidates to pick from. Then for time slot B he has 3 choices left (he doesn't want to interview the same person again), and C has 2 choices, with D having just one choice. Effectively we start at 4 and count our way down to 1. We will multiply these values out.
Overall, Bob has 4*3*2*1 = 24 different ways to arrange the candidate interviews. This is known as a permutation because order matters.
