黄皮OG诊断题数学6#（48页）求助

A marketing firm deterined that, of 200 households surveyed, 80 used neither Brand A nor Brand B soup, 60 used only Brand A soap, and for every household that used both brands of soap, 3 used only Brand B soap.

问:how many of the 200 households surveyed used both brands of soap?

答案:
15