gmat官方指南p21第六题，不明白怎么解

gmat官方指南p21第六题，不明白怎么解

200个调查对象中，80个两者都不用。剩下的120个中，60个只用A。那么用B的和两者都用的一共是60个。for every household that used both brands of soap, 3 used only brand B soap. 意思是用B的人数是两者都用的人数的三倍。60/4=15，即15个两者都用。

A MARKETING FIRM DETERMINED THAT, OF 200 HOUSEHOLDS SURVEYED,80 USED NEITHER BRAND A OR BRAND B SOAP,60 USED ONLY BRAND A SOAP, AND FOR EVERY HOUSEHOLD THAT USEDBOTH BRANDS OF SOAP, 3 USED ONLY BRAND  B SOAP. HOW MANY OF THE 200HOUSEHOLDS SURVEYED USED BOTH BRANDS OF SOAP?
回答：200个调查对象中，80个两者都不用。剩下的120个中，60个只用A。那么用B的和两者都用的一共是60个。for every household that used both brands of soap, 3 used only brand B soap. 意思是用B的人数是两者都用的人数的三倍。60/4=15，即15个两者都用。