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.
The answer is 480 units. You take the difference between the two numbers. I.E. 240- (-240)=480. When it comes to this situation think one person went 240 feet away and the other went 240 away. Neither of them are at the starting point so the answer can not be 0.