Explanation : From 15 minutes to 20 minutes, your starting point at 15 is 590 liters and at 20 you have exactly 500. Next you have 25 which is now at 410. So it's leaking 90 liters every five minutes, so of course dividing the two (90 ÷ 5) you get 18.
(a)² it increases for about the same time that it would decrease of course.
(b) The vat started at 860 liters before they started draining it.
Explanation : Well, all it takes is just working backwards, add 90 liters to the liters at 15 minutes and deduct five minutes from 15. Do this process until you arrive at 0. It's simple
