A marketing firm determined that, of 200 households surveyed, 80 used neither brand A nor brand B soap, 60 used only brand A soap, and for every househols that used both brands of soap, 3 used only brand B soap. How many of the 200 households surveyed used both brands of soap?

A. 15
B. 20
C. 30
D. 40
E. 45

The officeial guids says answer as A.15.

120 = 60 + 3x +x.

But i feel the answer as C.30
120 = 60 +3x - x. (As x is commonly used brand shouldn't it be subtracted.)

Please advice.

thanks
-Hari