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1. The speed of a motor-boat is that of the current of water as 36:5. The boat goes along with the current in 5 hours 10 minutes. It will come back in:

A. 5 hours 50 minutes

B. 6 hours

C. 6 hours 50 minutes

D. 12 hours 10 minutes

Solution:

Let the speed of the motor boat = 36x kmph and speed of current = 5x kmph.
The boat goes along with the current in 5 hours 10 minutes =

\frac{31}{6}

hour

\begin{aligned} Hence, Distance \\ = \frac{31}{6} \times (36 x + 5 x) \\ = \frac{41 x + 31}{6} k m \\ Speed of boat upstream \\ = 36 x + 5 x = 31 x kmph \\ Hence, time taken to come back \\ = \frac{41 x \times \frac{31}{6}}{31 x} \\ = \frac{41}{6} hours \\ = 6 hours 50 minutes \end{aligned}

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