Tanya prepared 4 different letters to be sent to 4 different addresses. For each letter, she prepared an envelope with its correct address. If the 4 letters are to be put into the 4 envelopes at random, what is the probability that only 1 letter will be put into the envelope with its correct address? 没有什么思路，脑袋一团浆糊，大牛请帮忙，谢谢。
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发表于 2010-10-7 22:00:19
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