A distillation system consisting of a reboiler and a total condenser (no column) is to be used to separate A and B from a trace of nonvolatile material. The reboiler initially contains 20 lbmol of feed of 30 mol% A. Feed is to be supplied to the reboiler at the rate of 10 lbmol/h, and the heat input is so adjusted that the total moles of liquid in the reboiler remains constant at 20. No residue is withdrawn from the still. Calculate the time required for the composition of the overhead product to fall to 40 mol% A. The relative volatility may he assumed constant at 2.50.