标题: prep math 概率题 [打印本页] 作者: zcooles 时间: 2010-8-8 10:38 标题: prep math 概率题
A certain junior class has 1,000 students and a certain senior class has 800 students. Among these students there are 60 sibling pairs, each consisting 1 junior and 1 senior. If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibiling pair?
A 3/40,000 B. 1/3,600 C. 9/2,000 D. 1/60 E. 1/15
答案是A， 但是不会做。
有牛人会做的帮忙解答一下哈~作者: min356478 时间: 2010-8-8 20:53
这个不算难题的，仔细看看
2 students selected will be a sibiling pair，就从从60pairs选一对，无顺序。
C60取1/(C1000取1*C800取1）作者: onedayling 时间: 2010-8-9 06:36
