A binary solution, containing 30 mole percent of A, is fed, at its boiling temperature, to a continuous distillation column, to produce a top and bottom product containing 90 mole percent and 10 mole percent of A, respectively. The vapour leaving the top of the column is condensed but not cooled; the reflux ratio is 6. The relative volatility, a , remains constant at 1.80. When the Murphree L-phase efficiency is 80%, calculate: (a) the number of plates required for the separation; (b) the minimum reflux ratio for the system.