Hello oexplooitable and welcome to MathHelpForum.

You should understand that this is not a homework service nor is it a tutorial service. Please either post some of your own work on this problem or explain what you do not understand about the question.

To get you started. In #20 just make a list of pairs that add to a multiple of five. Then count them.

In the next one, there are ways to divide eight people into two teams of four.