A tank contains 1,000 L of pure water. Brine that contains 0.05 kg of salt per litre of water enters the tank at a rate of 5 L/min. Brine that contains 0.04 kg of salt per litre 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. How much salt is in the tank? a. after t minutes? b. after one hour? c. eventually?