:(10 points) a brine solution of salt 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 the salt in the brine entering the tank is 0.2 kg/l, determine the mass of salt in the tank after t min.