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?
~$3 \over 8$~
~$1 \over 4$~
~$3 \over 16$~
~$1 \over 8$~
~$1 \over 16$~
生四次,总共有16中可能性。
生两男两女,排列组合;考虑两个女孩或男孩的出生顺序可能性,两个男孩或女孩有一方的顺序决定了,即决定了所有两男两女排列的可能性。
P(2,4)/16, 即3/8.
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