求助og 数学3.1 第6题

大宁子

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


求助这道题，看了解析，但是不明白为什么是60+x+3x，为什么是3x？ 求大神帮忙解析一下。

小熊橡皮糖

这种题画饼图来做。

“for every household that used both brands of soap, 3 used only Brand B soap.”
每个两种肥皂都用的家庭，都有三个只用B皂的家庭与其对应。即比例1：3

设用两种皂的家庭数为x，只用B的就是3x