How many four letter sub-sets are possible that contain either A or E but not both

Question

Four distinct letters are to be drawn from the set {A,B,C,D,E,F and G}. You are to create sub -sets out of these letters. Examples of a sub-set would be {A,C,D,F}, {E,B,D,G}etc. A sub-set is like grouping the letters into smaller sets. How many four letter sub-sets are possible that contain either A or E, but not both

Answer 1

If A is part of the subset, then the remaining 3 letters are needed to be drawn from the remaining 6 letters in the main set, but excluding E, so that means there are only 5 to choose from: 10 ways. Similarly for E, excluding A, so the total is 20.