Work Step by Step
A permutation from a group of items occurs when no item is used more than once and the order of arrangement makes a difference. (examples: arrangements, ordered lists, itineraries, schedules...) A combination from a group of items occurs when no item is used more than once and the order of items makes no difference. (examples: committees, subsets, groups, ...) ---- Order of selecting would not matter if you selected 4 actors from 20 for some (any) four roles. Here, however, order matters, as the role of "Mark" differs from the role of "Roger" and the other roles. So, you would be selecting an ordered list of actor names, with the first actor corresponding to Mark, the second to Roger, and so on. Permutations.