An item is on sale for 50% off the regular price of $3.18. a customer buys one and gives the cashier $5.00. the cashier gives the customer

The regular price is 3.18.
 50% off half the price, so you can either divide the regular price by 2 or multiply it by 0.50

3.18 / 2 = 1.59   ( 3.18 *0.50 = 1.59)

so the sale price is 1.59

now subtract the sale price from $5 to find the amount of change they got back:

5.00 - 1.59 = $3.14 change back

1.17+3.99a. you combine the terms with a to create 3.99 and the 1.17 stays because it has no like terms or a variable