A chemical manufacturing company has a 1000 k1 holding tank which it uses to control the release of pollutants into a sewage system. Initially the tank has 360 $kl$ of water containing 2 kg of pollutant per $kl$. Water containing 3 kg of pollutant per $kl$ enters the tank at the rate 80 kl per hour and is uniformly  mixed with the water already in the tank. Simultaneously water is released from the tank at the rate of 40 kl per hour. If p(t) is the amount of pollutant (in kg) in the tank at time t.