The first one is boy or girl, so two choices: 2
once the first one's gender is determined, only one of the other three children can be the same as the first one in gender, so 3 choices. As long as the first two children's gender is determined, no other choices. So:
2*P(1,3)/2^4
欢迎光临 国际顶尖MBA申请交流平台--TOPWAY MBA (http://forum.topway.org/)
