A 12,000 -liter tank of water is filled to capacity. At time t=0, water begins to drain out of the tank at a rate modeled by r(t), measured in liters per hour, where r is given by the piecewise-defined function

r(t) = { 600t/t+3 for 0≤t≤5
1000e −0.2t for t>5

Find the average rate at which water is draining from the tank between time t=0 and time t=8 hours