In a class of 35 students, twice the number if student who study maths study english and half the number of students who study both subjects study maths.
How many students study both subjects?
surely the number who study math is more than the number who study both, unless when you say "study math" you mean "study only math."
In that case, if b = # who study both e and m,
e = 2m
1/2 b = m
e+b+m = 35
solve that for b and you get 20 study both