Let
a1----------> price of box of cereal #

b1----------> price of box of cereal #

a2---------> size of box #
(in ounces,grams or some other unit)
b2---------> size of box #
(in ounces,grams or some other unit)
we know that
A rate is a comparison of two quantities that have different units
So
Step 1
Find the rate of the box of cereal #


Step 2
Find the rate of the box of cereal #


Step 3
Compare the rates
Whichever has the smaller rate is the best deal.
remember, to be able to compare both cereal boxes they must have the same unit of measure (grams with grams, ounces with ounces, etc.)