Create a table, DepartmentCodes, with three records and two fields in each record: The fields will be Department and Code. define a relationship like this:
YourTable::Departments = DepartmentCodes::Code
Define a value list of Deparments with the "use values from a field" option. Select DepartmentCodes::Code for the first field and DeparmentCodes::Department for the second field. You can hide the first field if you want.
Format YourTable::Departments as drop down list. Put DepartmentCodes::Department on the layout next to the drop down list.
When you select a department from the dorp down list, the department name will appear in the DepartmentCodes::Department field and the number will appear in your drop down field.