The number of gallons of water remaining in a swimming pool t minutes after the drains are opened and the pool begins emptying is give given by the function f(x)=40t^2-3000t+40,000 gallons. Between 3 and 5 minutes after the drains were opened, the pool emptied at an average rate of ___ gallons per minute. Find the average rate of change between t=3 and t=5