One or both

Problem 1 was solved by 70% of the pupils. Problem 2 was solved by 60% of them. Can you work out how many pupils took the exam?

Problem



A maths exam contained only two questions. Every pupil correctly answered at least one of the questions. 

Question one was correctly answered by 70% of the pupils. Question two was correctly answered by 60% of them.

Nine pupils correctly answered both questions.

How many pupils took the exam?