On the faculty of a university there are three professors named Smith: Adam Smith, Brett Smith, and Craig Smith. Draw a tree diagram to show the various ways in which the payroll department can distribute their paychecks so that each of them receives a check made out to himself or to one of the other two Smiths. In how many of these possibilities will (a) only one of them get his own check; (b) at least one of them get his own check?