This is the first problem which I solved:

You have 3 envelopes and 3 paychecks addressed to 3 different people. The paychecks are randomly inserted into the envelopes. What is the probability that at least one paycheck is inserted in the correct envelope.

The answer is: 4/3!= 2/3

Then I tried to make a general rule for the problem without success. Can anybody help me?

New problem will be: You have n envelopes and n paychecks addressed to n different people...