A typist types four envelopes and four letters. She places the letters in the envelopes at random. What are the chances that onl

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问题 A typist types four envelopes and four letters. She places the letters in the envelopes at random. What are the chances that only three letters are in their correct envelopes?

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答案zero

解析 (如果三个都装对了,那么第四个也一定装对了,所以没有这种几率。)
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