Bob is having a party and want to order some cake. there will be 57 guest and he wants to make sure that 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. first cake 20cm, serve 12 people and cost £13.50, second cake 25cm, serve 20 people and cost £18.75

To find out which cake is better for the price, we need to multiply the cake amounts by how many cakes of that kind we need. You need 5 20cm cakes, so we multiply 5 by the price of the 20cm cake (13.50) to get $67.50. You need 3 25cm cakes, so 3 x 18.75 = 56.25.

20cm cake: $67.50
25cm cake: $56.25

The 25cm cake is cheaper for more people.