A tank contains 50 gal of a solution of NaCl in water (20% wt). To wash the tank, we supply fresh water at 10 gal/min and drain the tank at the constant rate of 10 gal/min. Determine the concentration of salt in the tank as a function of time and the time till the tank is free of salt. The density of solution may be assumed independent of salt concentration and equal to that of water.