An acetone-air mixture containing 0.015 mole fraction of acetone has the mole fraction reduced to one-hundredth of this value by counter-current absorption with water in a packed tower. The gas flow rate is 4000 kg/hr.sqm of air and the water entering is 6250 kg/hr.sqm. For this system, Henry's law holds good and y = 1.75 x, where y is the mole fraction of acetone in the gas in equilibrium with a mole fraction x in the liquid. How many overall transfer units are required?