A tank contains 1000 l of pure water. Brine that contains 0. 05 kg of salt per liter of water enters the tank at a rate of 5 l/min. Brine that contains 0. 04 kg of salt per liter of water enters the tank at a rate of 10 l/min. The solution is kept thoroughly mixed and drains from the tank at a rate of 15 l/min.