A tank is initially filled with 1000 litres of brine, containing 0.15kg of salt per litre. Fresh brine containing o.25kg of salt per litre runs into the tank at the rate of 4L/s, and hte mixture (kept uniform by vigorious stirring) runs out at the same rate. Show that if Q (in kg) is the amount of salt in the tank at time t (in s) then dQ/dt = 1 - Q/250?