Solution:
$$\eqalign{
& BD\,{\text{for}}\,3\,{\text{years}} = {\text{Rs}}{\text{.}}\,1116 \cr
& BD\,{\text{for}}\,4\,{\text{years}} = \frac{{1116}}{3} \times 4 \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{. }}1488 \cr
& TD\,{\text{for}}\,4\,{\text{years}} = {\text{Rs}}{\text{.}}\,1200 \cr
& F = \frac{{BD \times TD}}{{BD - TD}} \cr
& \,\,\,\,\,\,\, = \frac{{1488 \times 1200}}{{1488 - 1200}} \cr
& \,\,\,\,\,\,\, = \frac{{1488 \times 1200}}{{288}} \cr
& \,\,\,\,\,\,\, = \frac{{124 \times 1200}}{{24}} \cr
& \,\,\,\,\,\,\, = \frac{{124 \times 100}}{2} \cr
& \,\,\,\,\,\,\, = 62 \times 100 \cr
& \,\,\,\,\,\,\, = {\text{Rs}}{\text{.}}\,6200 \cr} $$
⇒ Rs. 1488 is the simple interest on Rs. 6200 for 4 years
$$\eqalign{
& \Rightarrow 1488 = \frac{{6200 \times 4 \times R}}{{100}} \cr
& \Rightarrow R = \frac{{1488 \times 100}}{{6200 \times 4}} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{372 \times 100}}{{6200}} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{372}}{{62}} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 6\% \cr} $$
