The result of a survey of 100 families showed that 75 families used the towns swimming pool, 20 families used the town tennis courts, and 15 families used both the pool and courts. What fraction of the families surveyed used neither the pool nor the courts?

What do you think the answer is?

You need to make a Table with the information with a column for activity, # of families using the activity then a column with the fraction of who used the activity in relation to the told families.
Then ask yourself if they didn't use the activities then what piece of information do you need to find them? 75 20 or 15,
75 is pool, 20 tennis 15 both.
Good luck