12 oz = $1.09.
$1.09 divided by 12 = 0.09083.
20 oz = $1.99
$1.99 divided by 20 = 0.0995
The better buy is the 12 oz bottle because you pay less for the same amount. Because the $1.09 bottle was divided by 12, and the $1.99 was divided by 20, you get the amount each bottle costs if they only contained 1 oz. The better buy is the 12 oz bottle.