请教，一个manhattan数学题，

崇子

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?

		5/21
		3/7
		4/7
		5/7
		16/21

calvin1983

房间里面的有 a1a2a3 b1b2 c1c2  同字母表示sibling
则有  pro=(C13*C12+C13*C12+C12*C12)/C27=16/21  E

崇子

恩，谢谢楼上的。
其实我题目刚开始没读懂。
这个题目就相当于编号的三堆东西（类似于2个red balls，2个blue balls，3个white balls），然后要你从中选出不同的2个的Pro。。。
哎，题目读的我怪变扭的~~~