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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【正确答案】 1、正确答案：zero    
【答案解析】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。)