A group of friends ordered two pizzas, one with pepperoni and one with mushrooms and green peppers. A plain pizza costs 9 dollars. Each extra topping costs 1 dollar. How much did the two pizzas cost?

1 pizza = 9 dollars

1 topping = 1 dollar

1. Let's gather the information

2 Pizzas

One pizza is a pepperoni

The other is a mushroom and green pepper

2. Make an equation for the question

x9 + y1 = ?

x = number of pizzas

y = number of toppings

3. Multiply x and y by the number given

2 x 9 + 3 x 1 = ?

2 x 9 + 3 x 1 = 21

Which means, the two pizzas had costed 21 dollars.