In this exercise, we will develop an application for a small CD publishing
company. The owner wants the prices to be calculated so a customer would get a
discount if he orders more. To attract customers and encourage them to order
more, she has decided to fix the prices so that the customer would pay:
 $20/CD if he orders less than 20 units
 $15/CD if he orders less than 50 units
 $12/CD if he orders less than 100 units
 $8/CD if he orders less than 500 units
 $5/CD if he orders more than 500 units
