请教一题prep概率。急！！！

jillgu

哪位NN帮一下忙啊

 

谢谢啊

 

T prepared 4 difference 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?

 

Ans: 1/3

 

 

jillgu

我顿悟了。不好意思

嘻嘻

C(1,4)*C(1,2)*C(1,1)/P(4,4)