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 both brands of soap?fficeffice" />

A15  B20  C30  D40  E45

帮你顶

题目的关键在于，只用B肥皂的受访家庭是两者都用的3倍。已知条件，80户既不用A也不用B，60户只用A，则只用B的和两者都用的也是60户，得出两者都用的有15户。