The problem is as follows: Five paychecks and envelopes are addressed to five different people. The paychecks are randomly inserted into the envelopes. What is the probability that (a) exactly one paycheck is inserted in the correct envelope and (b) at least one paycheck is inserted in the correct envelope?

The answer for (I believe) part B is 45/120.

I just don't know how to get it.

Thanks for your help!