Discussion

If Rahul rows 15 km upstream in 3 hours and 21 km downstream in 3 hours, then the speed of the stream is

Answer: D

With \( b \) the still-water speed and \( r \) the stream's,

\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]

\[ b - r = \frac{15}{3} = 5 \text{ km/hr}, \qquad b + r = \frac{21}{3} = 7 \text{ km/hr} \]

\[ r = \frac{7 - 5}{2} = 1 \text{ km/hr} \]

Both trips take 3 hours, so the distances are proportional to the speeds. The 2 km/hr option is the raw difference, twice the current.

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