In class, students want to join sports. 15 people will join football, 24 people will join basketball, and 7 people will join both. How many people are there in the class?

Correct Answer: 30
There are 15 people who wish to join football, but 9 of those people also join basketball. By using the principle of inclusion and exclusion, we have: 15 people joining football + 24 people joining basketball – 9 people who will join both = 30 people total.