The present worth of a certain bill due sometime hence is Rs. 400 and the true discount is Rs. 20. What is the banker's discount?

$$\eqalign{ & BG = \frac{{{{(TD)}^2}}}{{PW}} \cr & \,\,\,\,\,\,\,\,\,\,\,\, = \frac{{{{20}^2}}}{{400}} \cr & \,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{.}}\,1 \cr & BG = BD - TD \cr & \Rightarrow 1 = BD - 20 \cr & \Rightarrow BD = 1 + 20 \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{.}}\,21 \cr} $$