Product a is a 12-ounce bottle of generic popcorn oil that sells for $1.39. product b is a 24-ounce popcorn oil bottle that costs $2.79. which is the better buy?

Simply divide the price by the number of ounces to get the price per oz.

$1.39 ÷ 12 = about $.1158 cents per oz
$2.79 = 24 = about $.11625 cents per oz.

By a very small amount the $1.39 bottle is a better deal.  

You could also think that $1.39 for 12 oz is $2.78 for 24 (I doubled both numbers)
$2.78 for 24oz  is a drop cheaper than $2.79 for 24 oz  so $1.39 is a slightly better buy.