bob is having a party and wants to order cakes. There will be 57 guests and he wants everyone to get a slice of cake. he wants to pay as little as possible.

How much will he have to pay for the cakes?

1st cake is:

20cm cake.

Serves.
12 people
Cost
£13.50

2nd cake

25cm cake

Serves.
20 people
Cost
£18.75.

Please can somebody help me work this out.