A refinery has two smelters that extract metallic iron from iron ore. Smelter A processes 1000 tons of iron ore per hour and uses 7 megawatts of energy per hour. Smelter B processes 2000 tons of iron ore per hour and uses 13 megawatts of energy per hour. Each refinery must be operated at least 8 hours per day and, of course, no more than 24 hours. If the refinery must process at least 30,000 tons of iron ore per day, how many hours should each smelter operate in order to expend as little energy as possible?