求助！一道关于概率的数学难题

127HLLJ

In a room filled with 7 people, 4 people have exactly 1 sibling in the room and 3 people have exactly 2 siblings in the room. If two individuals are selected from the room at random, what is the probability that those two individuals are NOT siblings?

答案是21分之16

哪位老师能帮我解答一下？

emicywwq

根据题意 可以把7个人标记为A1 A2 B1 B2 C1 C2 C3 A1和A2； B1和B2； C1 C2和C3是兄弟姐妹 从7人中任选两人的可能性是C7 2=6*7/2=21 不是姐妹的可能性为1.A1、A2；B1、B2中任选一人 2*2=4 2.A氏和C氏中各选一人 2*3=6 3.B氏和C氏中各选一人 2*3=6 总：4+6+6=16 所以 答案为16/21

127HLLJ

太感谢你了！！我弄懂了！！！