Discussion

Sahil can row 3 km against the stream in 20 minutes and he can return in 18 minutes. What is rate of current ?

Answer: A

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

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

Convert each time to hours and divide into the 3 km.

\[ b - r = \frac{3}{20/60} = 9 \text{ km/hr} \]

\[ b + r = \frac{3}{18/60} = 10 \text{ km/hr} \]

\[ r = \frac{10 - 9}{2} = \frac{1}{2} \text{ km/hr} \]

The current's speed is half the difference of the two, not the difference itself. Sahil rows at 9.5 km/hr in still water.

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