[求助]OG12里的数学

NN们请帮个忙，下面是OG12里数学的题，我怎么搞不清“and for every household that used both brands of soap, 3 used only Brand B soap”这句话的逻辑了，多谢拉！！！

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) 40
(E) 45

OG11 诊断测试里面也有这道题。

那句话是比较别扭，主要是大陆英文老师从来不教给学生如此大胆的句式。

其意思是：相对于每个同时使用A/B两个品牌的家庭，有3个只使用B品牌的家庭。设前者为X，后者即3X。

200-80=60+x+3x

4x=60

x=15

KEY=A

under "3.1 Quantitative Sample Questions"

哦，豁然开朗，谢谢啦：）