Mental Arithmetic (Cashier Problem)

The Tale

You work in a store and make change for customers. You are presented with with the total cost of items purchases. The customer gives you cash and you must make change. (The difference between the total cost and the amount tendered.)

Project #1

Create an interactive program to practice simple mental arithmetic:

Project #2

Covert Project #1 to a GUI.

Project #3

Turn it into a game by timing how long it takes to perform 10, 20, ... problems.