A brine solution flows at a constant rate of 4 L/min into a large tank that initially held 100 L of pure water. The solution inside the tank is kept well stirred and flows out of the tank at a rate of 3 L/min. If the concentration of salt in the brine entering the tank is 0. 2kg / L, determine the mass of the salt in the tank after t time. When will the concentration of salt in the tank reach 0.1 kg/L?