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 NaCl solutions is given by the empirical equation

ρ = ρ0 C 0:925ω

where ρ is the solution density (g=cm³), ρ0 is the density of fresh water (1 g=cm³) and w is the mass fraction of NaCl. (Hint: Requires numerical integration.)