20. Find how many ways two numbers can be selected from the numbers 1, 2, ... , 8, 9 so that their sum is:

(c) divisible by 3, (d) divisible by 5, (e) divisible by 6.

22. Nine players are to be divided into two teams of four and one umpire.

(a) In how many ways can the teams be formed?

(b) If two particular people cannot be on the same team, how many different combinations are possible?

23. Four adults are standing in a room that has five exits. Each adult is equally likely to leave the room through any one of the five exits.

(d) What is the probability that no more than two adults come out the same exit?

There will be more to come :-(