A system consisting of a still-pot and a total condenser is used to separate A and B from a trace of nonvolatile material. The still-pot initially contains 20 lbmol of feed of 30 mol% A. Feed of the same composition is supplied to the still-pot at the rate of 10 lbmol/h, and the heat input is adjusted so that the total moles of liquid in the reboiler remain constant at 20. No residue is withdrawn from the still-pot. Calculate the time required for the composition of the overhead product to fall to 40 mol% A. Assume α = 2.50.