Question

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

Answer 1

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.

Answer 2

rate
cost/amount
3/24=1/8, 1 doller per 8 botttles

4/36=1/9, 1 dollar=9 bottles

9>8
9-8=1
if by 'better buy' you mean cheaper price then
24 or $3 is better by 1 bottle per bottle