If you have 4 lb of cortland apples and each costs $1.20 you multiply them and get $4.80 for the 4 lb, and because you can spend $7.20, you subtract the $4.80 of the cortland apples and you still have $2.40. Then you divide this by $1.20 that is the price of 1 lb of gala apples and you get 2 that is the number of lb of gala apples you can buy.
