问答题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?
问答题
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?
参考解析
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。
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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