The true discount on Rs. 2562 due 4 months hence is Rs. 122. The rate percent is:

$$\eqalign{ & {\text{P}}{\text{.W}}{\text{.}} = Rs.\left( {2562 - 122} \right) \cr & = Rs.2440 \cr & \therefore {\text{S}}{\text{.I}}{\text{.}}\,{\text{on}}\,{\text{Rs}}{\text{.}}\,{\text{2440}}\,{\text{for}}\,{\text{4}}\,{\text{months}}\,{\text{is}}\,{\text{Rs}}{\text{.}}\,{\text{122}}{\text{.}} \cr & \therefore {\text{Rate}} = \left[ {\frac{{100 \times 122}}{{2440 \times \frac{1}{3}}}} \right]\% = 15\% \cr} $$