A pet store is having a pet fish sale. Mrs. Smith bought koi and paid with a​ $50 bill. Write an algebraic expression to represent how much change Mrs. Smith got back.

Arestaurant offers delivery for their pizzas. the total cost the customer pays is the price of the pizzas, plus a delivery fee. one customer pays $23 to have 2 pizzas delivered. another customer pays $59 for 6 pizzas. how many pizzas are delivered to a customer who pays $86?