Target sells 24 bottles of water for $3, and 36 bottles of water for $4. which is the better buy and by how much?

You should NEVER buy plastic water bottles! only the reusable ones, but if you must know, it's cheaper to buy the 24 pack, because if you divide 24 by $3 you get 0.8 for the cost of each water bottle.
If you divide 36 by $4 you get 0.9 for the cost of eact water bottle.