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:

Answer: Option C

1 hour 45 minutes = $$1+\frac{45}{60}$$  = $$\frac{7}{4}$$ hours.
Speed of the motorboat up-stream,
$$\begin{array}{rl}& =\frac{\text{Distance}}{\text{Time}\text{Taken}}\\ & =\frac{56\text{km}}{\frac{7}{4}\text{hours}}\\ & =\frac{56\times 4}{7}\\ & =32\text{kmph}\end{array}$$
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.