A tank with a capacity of 4000 L is full of a mixture of water and chlorine with a concentration of 0.005 g of chlorine per liter. In order to reduce the concentration of chlorine, fresh water is pumped into the tank at a rate of 40 L/s. The mixture is kept stirred and is pumped out at a rate of 100 L/s. Find the amount of chlorine in the tank as a function of time. (Let y be the amount of chlorine in grams and t be the time in seconds.)