Hi. I have received this problem as practice for a math competition and could not solve it. I spent about an hour getting nowhere. Any ideas on what to do?

There are 25 students in Mrs. Best's class. Thirteen are not members of the math club and 17 are not members of the chess club. There is at least one student who is a member of both clubs, at least one student in Math Club but not the Chess club and at least one student in Chess club but not Math club.

Determine the average number of students who could be members of both clubs.