To compare the cost, you can calculate the cost per ounce of cereal. This means you will have to divide the cost by the total number of ounces. The cereal that costs the least per ounce will be the best buy.
$4.79 / 18oz = $0.27/oz
$3.89 / 21oz = $0.19/oz
$6.49 / 32oz = $0.20/oz
The 21-ounce size is has the cheapest price per ounce, so this is your best buy.
