How to write a proportion for this?

Question

Three cartons of juice cost $4.77. Find the cost of 8 cartons.

If a pizza with a diameter of 12 inches costs $10.99, based on area, how much should a 15-inch pizza cost.

Answer 1

Since you are essentially calculating a proportion on the effect of areas, you must square the diameters to set up the proportion equation in quadratic format:

Let x = cost of the 15 inch pizza

12²/15² = 10.99/x

So

144x = 225*10.99

144x = 2472.75

x = $17.17 (the cost of the larger pizza)
.

Answer 2

You just say 3/$4.77 and you get .63 so .63 times 8 is $5.03

12/$10.99 so $1.09 per inch so a.09 times 15 is $16.38