A 30-ton engine exerts a constant horizontal force of 40(10^3)-lb on a train having three cars that have a total weight of 250-tons. If the rolling resistance is 10-lb per ton for both the engine and cars, Determine how long it takes to increase the speed of the train from 20 ft/s to 30 ft/s. What is the driving force exerted by the engine wheels on the tracks?