A tank contains 1,000L of pure water. Brine that contains 0.05kg of salt per litre of water enters the tank at a rate of 5L/min. Brine that contains 0.04kg of salt per litre of water enters the tank at a rate of 10L/min. The solution is kept thoroughly mixed and drains from the tank at a rate of 15L/min. How much salt is in the tank?

a) after t minutes?

b) after one hour?

c) eventually?