2. 外语
  3. There are 30 pairs of socks in a drawer, 60％ of the socks are red and the rest are blue. What is the minimum pairs of socks that

There are 30 pairs of socks in a drawer, 60％ of the socks are red and the rest are blue. What is the minimum pairs of socks that

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问题 There are 30 pairs of socks in a drawer, 60％ of the socks are red and the rest are blue. What is the minimum pairs of socks that must be taken from the drawer without looking in order to be certain that two pairs of blue socks have been chosen?
选项 A、12
B、2
C、14
D、18
E、20
答案E
解析 30双袜子中红色的有30×60％=18双，其余的都是蓝色共12双。
如果已经取出了18双袜子，那么在最坏的可能下，这18双袜子都是红色的，没有一双是蓝色；但是在这种情况下，如果再多取一双袜子，那么必有一双是蓝色的。也就是至少取19双袜子才能够保证有一双必定是蓝色的。
显然，至少取20双袜子才能够保证有两双必定是蓝色的，所以E为正确答案。
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