Can anyone explain this?

Can anyone explain this?

If your order is $20 or under, you receive a discount of $5. If your order more than $20 up to $40 (inclusive), you are given a discount of $10. If your order is more than $40, you are given a discount of $10 plus an additional discount equal to 50% of the amount that your order is over $40.

(a) Let x be the amount that you spend (prior to the discount) at the grocery store and let D(x) be the discount that is applied based on your bill. Find a piece-wise function for D(x).

b. Graph D(x)