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?
~$3 \over 8$~
~$1 \over 4$~
~$3 \over 16$~
~$1 \over 8$~
~$1 \over 16$~
二项式分布C(n,k)*p^k*(1-p)^(n-k)
登录 或 注册 后可以参加讨论