Bob is having a party and wants to order some cakes.
there will be 57 guests and he wants to make sure everyone gets a slice of cake.
he wants to pay as little as possible for the cakes and for them to be the same size
how much will he have to pay for the cakes?

Bob can buy a cake that costs 13.50 Pounds and serves 12 people. It means that he needs 5 cakes of that type ( 5 * 12 = 60 > 57, so everyone gets a slice ). It would cost totally: 5 * 13.50 = 67.50 Pounds. Another cake that Bob can buy costs 18.75 Pounds and it serves 20 people. Now he would need 3 cakes ( 3 * 20 = 60 > 57 ). So 3 * 18.75 = 56.25 Pounds. And because Bob wants to pay as little as possible for the cakes, he would choose the 2nd cake. Bob will have to pay 56.25 Pounds.

Subtracting is the opposite of adding