A motorboat in still water travels at speed of 36 kmph. It goes 56 km upstream in 1 hour 45 minutes. The time taken by it to cover the same distance down the stream will be:

Solution (By Examveda Team)

1 hour 45 minutes = $$1 + \frac{{45}}{{60}}$$  = $$\frac{7}{4}$$ hours.
Speed of the motorboat up-stream,
$$\eqalign{ & = \frac{{{\text{Distance}}}}{{{\text{Time}}\,\,{\text{Taken}}}} \cr & = \frac{{56\,{\text{km}}}}{{\frac{7}{4}{\text{hours}}}} \cr & = \frac{{56 \times 4}}{7} \cr & = 32\,{\text{kmph}} \cr} $$
Let the speed of the current = x kmph
Hence, 36 - x = 32
Or, x = 36 - 32 = 4 kmph
Speed of boat down the stream = 36 + 4 = 40 kmph.
∴ Time taken to cover 56 km at 40 kmph = $$\frac{{56}}{{40}}$$ = $$\frac{7}{5}$$ hours
or 1 hours 24 minutes.