probability - urgent help please

I'm having lots of problems with probability questions.

Could you help me solve this? Thank you.

**4 different letters are to be sent to 4 different addresses. For each letter there is an envelope with its correct address. If the 4 letters are to be put into the 4 envelopes at random, what it the probability that ony 1 letter will be put into the envelope with its correct address?**

I thought each letter has a 1/4 chance of being put in the right envelope, and a 3/4 chance of being put in the wrong envelope, so

P=(1/4)(3/4)(3/4)(3/4)

what am I doing wrong?

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