Venn Diagram vs. Formula [Grp 1 + Grp 2 - Both + Neither ]

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 household 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: 4...