A saturated vapour mixture of maleic anhydride and benzoic acid containing 10 mol% acid is a by-product of the manufacture of phthalic anhydride. This mixture is distilled continuously at 13.3 kPa to give a product of 99.5 mol% maleic anhydride and a bottoms of 0.5 mol% anhydride. Using the data below, calculate graphically the number of theoretical plates needed using an L/D of 1.6 times the minimum Reflux ratio Rm.