For this question, I have done in this manner. C=Chemistry, B=Biology, A=Both
120 = A+B+C,
2A = B+C,( substituting equation)
120 = A+2A
120 = 3A
40 = A
First: C and B represent only chemistry and biology respectively. The only part is important - otherwise the equation is wrong.
Also, your second equation should be A = 2(B + C) in that case.
So, I have made a wrong equation?
Yes.