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?
~$3 \over 8$~
~$1 \over 4$~
~$3 \over 16$~
~$1 \over 8$~
~$1 \over 16$~
生死次孩子,每次都有可能生出男或者女,2*2*2*2=16
生出2女2男的可能性:
BBGG
BGBG
BGGB
GGBB
GBGB
GBBG
总6种
所以是6/16=3/8
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