Exercise - Create a billing calculator
In this exercise, you build a small billing calculator for a store. You organize your tax and total calculations into their own file under a Billing namespace, then bring that namespace into your main program with a using directive. You practice defining methods with parameters and return values, giving a parameter a default value, overriding that default with a named argument, and calling methods through the class name they belong to.
Launch the exercise and follow the instructions.
