Discussion
Speed of a boat in standing water is 9 kmph and the speed of the stream is 1.5 kmph. A man rows to a place at a distance of 105 km and comes back to the starting point. The total time taken by him is:
Answer: D
Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b + r = 10.5 \text{ kmph}, \qquad b - r = 7.5 \text{ kmph} \]
\[ t = \frac{105}{10.5} + \frac{105}{7.5} = 10 + 14 = 24 \text{ hours} \]
Using the still-water speed for both legs would give \( \tfrac{210}{9} = 23.3 \) hours - less than the truth, because the extra time lost upstream outweighs the time saved downstream.
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