If the true discount on a certain sum due 6 months hence at 15% is Rs. 120, what is the banker's discount on the same sum for the same time and at the same rate?

$$\eqalign{ & B.G. = S.I.\,on\,T.D. \cr & = {\text{Rs}}{\text{.}}\,\left( {120 \times 15 \times \frac{1}{2} \times \frac{1}{{100}}} \right) \cr & = {\text{Rs}}{\text{.}}\,9 \cr & B.D. - T.D. = Rs.\,9 \cr & B.D. = {\text{Rs}}{\text{.}}\,\left( {120 + 9} \right) \cr & \,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{.}}\,129 \cr} $$