Question

Suppose that a school band contains 10 students from the freshman class,20 students

from the sophomore class.30students from the junior class and 40 students from the senior class. If 15 students are selected at random from the band, What's the probability that at least one student will be selected from each of the four classes?
为什么不能 10*20*30*40*（96C11）/（100C15） 就是说为什么不能先定好每个class选出的那一人，然后再从剩下的96个人里面选11个？ 这样算概率是83.3 而答案是0.793= =

Answer 1

至少有一个的概率应该是 100% 减去 一个人都没有的概率。

如： 第一个班至少有一个的概率是 1 - （在后3 个班里面选15个人）/ 40*30*20*10=9.09%