Water flows in and out of a storage tank. A graph of the rate of change $r(t)$ of the volume of water in the tank, in liters per day, is shown. If the amount of water in the tank at time $t = 0$ is 25000 L, use the Midpoint Rule with $n = 4$ to estimate the amount of water (in liters) in the tank four days later.

Amount of water = L