Sorry got wrong calculation. Questions said "every household that used both brands of soap, 3 used only Brand B soap."
So if Say x people used both brands, then 3*x = 3x people used only brand B.
Then total number of people using brand B = 3x + x = 4x
So As per Set theory
120 = 60 + 4x - x
=> x = 20
Brother u are close but u made a silly mistake3x people used only brand B.
so it is
120=60(only A)+3x(Only B)+x(Both)
Ans is 15