Product a is a 12 ounce bottle of generic mouthwash that sells for $1.39. product b is a 24 ounce bottle of mouthwash that costs $2.79. whuch is the better buy?

If you bought 2 Product A that would be 24 ounces for 2.88.Unless A is better quality or something, Product B is cheaper, therefore being the better buy.

Well let's see. Since product B is twice the amount of of product A, we'll have to see if product A is still cheaper if you get two bottles.

Product A(12 oz.)=$1.39 so just multiply the price by 2 
(1.39)*2=2.78
So if you bought 2 bottles of Product A it would be $2.78, which is just one cent cheaper than getting the 24 oz bottle that is $2.79.

Therefore, Product A would be the better buy, but not by very much lol.