A cistern (water tank) contains 200 litres of pure water. Brine (salty water) that contains 0.02 kg/L of salt is added at a rate of 3 L/min. Brine from a second source with 0.04 kg/L of salt is added at a rate of 1 L/min. Assume that the cistern is instantaneously well-mixed. The cistern is drained at a rate
of 4 L/min. If Q(t) is the amount of salt (in kg) at time t (in min), how much salt is in the cistern after t minutes?