An 18-ounce box of wheat cereal costs $3.60 A 12-ounce box of oat cereal cost $3.00

You first want to divide 3.60 by 18
3.60/18 = 0.2
This means that it would be 0.20 per ounce.

3.00/12 = 0.25
This means that it would be 0.25 per ounce.

18 ounce box is cheaper by ounce.