一道数学题

kimberleyzh

有一道题，把自己搅晕了，望懂得人解下。
A certain club has 10 members, including harry. one of the 10 members is to be chosen at random to be the president, one of the remaining 9 is to be chosen at random to be the secretary, and one of the remaining 8 members is to be chosen at random to be the treasurer. what is the probability that Harry will be either the member chosen to be the secretary or the member chosen to be the treasurer?

kimberleyzh

把题放上来后就想明白了。Harry 被选为 secretary 和 treasurer 是相对独立的，应该是两种概率的和，即9/10*1/9+9/10*8/9*1/8=1/5
总觉得概率不应该这么高，10个人呢。