GMAT
题目
Carcass has 4 different letters to be sent to 4 different addresses. For each letter, he prepared an envelope with its correct address. He then put 4 letters into the 4 prepared envelopes at random.
Quantity A
Probability that only 1 letter was put into the envelope with its correct address
Quantity B
解析
把正确的信放入第1个信封，给第2个信封放3个选项，其中只有2个是错误的，再给第3个信封放只有1个错误的选项。当第1个信封放对时，总情况数为 2×1=2
对4个信封重复此操作：2+2+2+2=8
总情况数 = 4! = 24
概率 = 8/24 = 1/3 < 2/5
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