请教一道概率题

童言无忌

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：1/3

请问这道题怎么做啊？请NN们帮忙解答。十分感谢！