Solution:
Let the speed of the current be x kmph
Then speed of the boat in still water = 5x
$$\eqalign{
& \therefore {\text{Downstream speed}} \cr
& {\text{ = }}\left( {5x + x} \right) = 6x\,kmph \cr
& {\text{Upstream speed}} \cr
& {\text{ = }}\left( {5x - x} \right) = 4x\,kmph \cr
& {\text{Now, }} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{13}}{\text{.2km}}\,\,\,\, \cr
& {\text{A}}\overline {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} {\text{B}} \cr
& {\text{According to question,}} \cr
& {\text{1}}{\text{.1}} \times {\text{6}}x = 13.2 \cr
& \Rightarrow 6.6x = 13.2 \cr
& \Rightarrow x = \frac{{13.2}}{{6.6}} \cr
& \therefore x = 2\,kmph \cr
& \therefore {\text{Upstream speed}} \cr
& {\text{ = 4}}x = 4 \times 2 = 8\,kmph \cr
& \therefore {\text{312 minutes}}\, \cr
& = 5\frac{1}{5}\,hours \cr
& = \frac{{26}}{5}\,hours \cr
& \therefore {\text{Required distance travelled upstream}} \cr
& {\text{ = Speed }} \times {\text{Time}} \cr
& {\text{ = 8}} \times \frac{{26}}{5} = 41.6\,km \cr} $$
