Yes I love this problem so good
So,
If you do some simple division, you will find how much each cm costs in each cake.
20cm cake = 67.5 cents/cm
25cm cake = 93.75 cents/cm
Assuming that you cannot order fractions of cakes, the best deal would be to order 5 of the 20cm cakes.
Cost = 13.50 * 5 = $67.50
You will have to pay $67.50 for the cakes if you want the best deal.
Multiply the top one by 3 for x, so you can eliminate x and find y
then plug in the y into one of the equations to find x
