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GMAT 的OG里quantitative review中Problem Solving 里第160题

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GMAT 的OG里quantitative review中Problem Solving 里第160题
a couple decides to have 4 children. If they succeed in having 4 children and each child is equally likely to be a boy or a girl, what is the probability that they will have exactly 2 girls and 2 boys?
求中文解题思路,谢谢!
4个孩子的性别共有2^4种可能性,而2男2女的概率是C42(角标上2下4)*2,即从4个里边随机选两个是同样的性别、其余两个是另外的性别,乘以2。结果是3/4