Recently I was in a department store, in the checkout line, when I overheard this conversation between the clerk and another customer:

Customer: "This item is 40% off the regular price and the sale items are an extra 40% the sale price".

Clerk: "That's correct."

Customer: "So it should be 80% off. It's not ringing up that way".

Clerk: "It's not actually 80% off. It's ringing up correctly".

The customer was irate, and a manager had to be called to explain why the item was not actually 80% off. Answer the following questions:

How would you explain this to a customer if you were in the manager's position? Give an example to illustrate your explanation.