題目是這樣子的: 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 圖片上面是我的想法: 令A表是他有到學校 R表示他有收到信 當他到學校時，收到信的機率就是1/3 而如果他沒到學校，就不會收到信 但解答的想法卻是下面的那樣 跟我的想法似乎相反 想問我的想法錯在哪呢？ --

※ 發信站: 批踢踢實業坊(ptt.cc) ◆ From: 116.59.247.9