6. 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
Arithmetic Operations on rational numbers
Since it is given that 80 households use neither
Brand A nor Brand B, then 200 – 80 = 120 must
use Brand A, Brand B, or both. It is also given
that 60 households use only Brand A and that
three times as many households use Brand B
exclusively as use both brands. If x is the number
of households that use both Brand A and
Brand B, then 3x use Brand B alone.
怎么也弄不明白（1）3倍的关系在题目中怎么体现的？（2）60户只用A牌子，怎么会包含了即用A牌子，又用B牌子的，题目中的only难道不起作用？ | 
