A saturated liquid mixture containing 60 mole % benzene and 40 mole % toluene is to be distilled continuously into a distillate product containing 90 mole % benzene and the bottom product containing 4 mole \% benzene. The fractional distillation column will operate at approximately constant pressure of 1atm. The reflux ratio is 3 . How many theoretical plates must the column have if the feed is introduced into the eighth plate? Equilibrium data are