The 12 oz for $2.36 is cheaper than the 18 oz for $3.65. The price per oz for the 12 oz is 20 cents. The price per oz for the 18 oz is also 20 cents. But if you go past rounding to the nearest cent you find the price per ounce for the 12 oz is 19.667 cents while the price per oz for the 18 oz is 20.2778 cents per oz.
