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

Well what you would have to do is figure out how many cakes you would need out of both.
12*5=60 and 20*3=60
no matter what there would be three pieces left over.
After you do that you would have to figure out how much they would cost so you would have to do 5*13.50 and 20*18.75 which equals 20*18.75= 56.25 and 5*13.50=  67.50
so if he wanted to pay little as possible it would be best to buy the 25cm which servers 20 and all together it would equal 56.25