请教大家一道prep数学题谢谢>0<

zuozhuo1993

prep数学 PS2-116
A certain manufacturer of cake, muffin, and bread mixes has 100 buyers, of whom 50 purchase cake mix, 40 purchase muffin mix, and 20 purchase both cake mix and muffin mix.  If a buyer is to be selected at random from the 100 buyers, what is the probability that the buyer selected will be one who purchases neither cake mix nor muffin mix?

数学宝典答案是50+40-20=70  （100-70）/100=0.3
我理解的是50人买cake
                 40人买muffin
                 其中20人是两个都买了
                那么求两者都没买的人数不应该是100-50-40=10么 所以概率应该是10/100=0.1啊
是我脑子短路了么T~T 求解答求帮助~~~~~~~~~~~~~谢谢大家~~~~~~~~~~~~~~~~·

zuozhuo1993

help~please~~~··

zuozhuo1993

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1odyssey3

买cake和买muffin 是有交集的啊，所以买了cake或者买了muffin的人加起来总和为50+40-20=70
买了cake或者买了muffin的人 相反 就是既没买cake又没买muffin的 为100-70=30