A well-stirred tank of volume V= 2m^3 is filled with brine, in which the initial concentration of NaCl at t=0 is Co=1 kg/m^3. subsequently, a flow rate of Q= 0.01 m^3/S of pure water is fed steadily to the tank, and the same flow rate of brine leaves the tank through a drain. Calculate the time needed for the NaCl concentration to fall to a final value of cf=0.0001 kg/m^3 within the tank.