7. There are 30 pupils in my class. 20 pupils like Maths and 18 pupils like English. Twice as many pupils like both subjects as like neither of them. How many pupils like only Maths?
A 20
B 16
C 12
D 8
E 4
7. E Let the number of pupils who like neither subject be $x$. Hence the number who like both subjects is $2 x$. Therefore the number of pupils who like only Maths is $20-2 x$ and the number who like only English is $18-2 x$. Since there are 30 pupils in my class, we have $(20-2 x)+2 x+(18-2 x)+x=30$ and hence $38-x=30$. This has solution $x=8$ and hence the number of pupils who like only Maths is $20-2 \times 8=4$.