题目是这样子的: The probability that,on the weekday,the college will receive letters addressed to Dr.A is 1/3. Dr.A,who arrives earlier than any of his colleagues,begins the day by collecting his mail. He has told me that there is 40% chance that he will attend college today;and I have noticed that there are no letters in his pigeon hole.In view of there being no mail in his box,what is the probability that he is attending today? http://miupix.cc/pm-6UFKAJ 图片上面是我的想法: 令A表是他有到学校 R表示他有收到信 当他到学校时，收到信的机率就是1/3 而如果他没到学校，就不会收到信 但解答的想法却是下面的那样 跟我的想法似乎相反 想问我的想法错在哪呢？ --

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