请教Prep一道数学题，多谢！

shirky2009

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?

答案是：1/3

请帮忙写一下方程式，多谢。

rickwry

2^3/P(4,4)

francesco

2*4/P(4,4)

shirky2009

多谢，这题不明白的地方主要是看到: " If the 4 letters are to be put into the 4 envelopes at random" , 这句话是否有可能 1 envelop 装4封信？

shaan

For each letter, she prepared an envelope with its correct address. 人家说这个意思就是不可能一个信封放几个信件。

tianle312

这道我也不大懂， 请问分子的8是如何得到的呢

shaan

你这样想好了：首先选一个正确的信放入信封，有四种可能吧，然后剩余的三个都不能正确，一共有两种可能，则4×2＝8.