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A marketing firm determined that, of 200 households [#permalink]
28 Apr 2008, 17:14
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A marketing firm determined that, of 200 households surveyed, 80 used neither Brand A nor Brand B soap, 60 usd 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. 40 E. 45
The equation also works, but you need to know how to use the diagram as well. Try using the equation when you have three different groups instead of just two. It won't work nearly as easily as a picture.