The banker's gain of a certain sum due 3 years hence at 10% per annum is Rs. 36. What is the present worth ?

$$\eqalign{ & {\text{T}} = 3\,{\text{year}} \cr & {\text{R}} = {\text{10}}\% \cr & {\text{TD = }}\frac{{{\text{BG}} \times 100}}{{{\text{TR}}}} \cr & \,\,\,\,\,\,\,\,\,\,\,\, = \frac{{36 \times 100}}{{3 \times 10}} \cr & \,\,\,\,\,\,\,\,\,\,\,\, = 12 \times 10 \cr & \,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{.}}120 \cr & {\text{TD = }}\frac{{{\text{PW}} \times {\text{TR}}}}{{100}} \cr & \Rightarrow 120 = \frac{{{\text{PW}} \times 3 \times 10}}{{100}} \cr & \Rightarrow 1200 = {\text{PW}} \times 3 \cr & {\text{PW}} = \frac{{1200}}{3} \cr & \,\,\,\,\,\,\,\,\,\,{\text{ = Rs}}{\text{.}}400 \cr} $$