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1. 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:

A. 2 Hours 25 Minutes

B. 3 Hours

C. 1 Hours 24 Minutes

D. 2 Hours 21 Minutes

Solution:

1 hour 45 minutes =

1 + \frac{45}{60}

\frac{7}{4}

hours.
Speed of the motorboat up-stream,

\begin{aligned} = \frac{Distance}{Time Taken} \\ = \frac{56 km}{\frac{7}{4} hours} \\ = \frac{56 \times 4}{7} \\ = 32 kmph \end{aligned}

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.

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