This is Zi Heng Lim's solution:

70% + 60% = 130%
130% - 100% = 30%
30% = 9 pupils
100% = 9/30 * 100
= 30 pupils.

30 pupils took the exam.

Andrei Lazanu, age 12, School 205, Bucharest, Romania solved this problem using a Venn diagram.

Venn diagram : A intersects with B.

Prateek Mehrotra, James Wakefield, Alan Rowan, Jenny Cook and Robert Haynes also sent in good solutions.