When given a question where finding/using a formula varies depending on the case, we should use cases. Break each question down into scenarios with 1 set in variable, then add all the cases you need that are mutually exclusive, if needed subtract the mutual elements and that is your new answer.
Example questions
pg 240 question 20. How many even three digit numbers from the set: {3,4,5,6,7} begin with a 4? you need to check 2 cases: [][][4] 3 4 [][][6] 3 3
3*4 + 3*3 = 21