Salt for salting winter roads is being unloaded into a storage garage at a rate of 10 ft3 per second. The salt moves from the truck up a conveyor belt, until it drops from the top of the conveyor belt forming a pile of salt that's shaped like a cone. The height of the pile of salt is always twice its radius. At what rate is radius of the pile increasing when its height is 5 ft? (Note: The volume of a cone of radius r and height h is 3 r2h.)