Mathematics, Science and English
In a certain school, the students have to study Mathematics, Science or English. Each pupil must study at least one of the three subjects. Among a group of 40 pupils, 20 are studying Mathematics, 22 Science, 28 English; 12 are studying Maths and Science, 14 are studying Science and English and 15 are studying Mathematics and English. How many pupils are studying all three subjects?