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. Answer: Bob will have to pay 56.25 Pounds.
