Froma bag containing 12 identical blue balls, y identical yellow balls, and noother balls, one ball will be removed at random. If the probability is less than 2/5 that the removed ball willbe blue, what is the least number of yellow balls that must be in the bag?
17
18
19
20
21
注意at least,不包含18,所以最少19个
怎么算出来y>18?
12/(12+y)<2/5
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算出来y>18,那么最小的y是19
at least
12/(12+x)18
至少19
拿出蓝球的概率是12/(12+y)18,所以最少要19个黄球