Discussion
In one hour, a boat goes 11 km/hr along the stream and 5 km/hr against the stream. The speed of the boat in still water (in km/hr) is:
Answer: C
Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b = \frac{(b + r) + (b - r)}{2} = \frac{11 + 5}{2} = 8 \text{ km/h} \]
The stream is \( \tfrac{11-5}{2} = 3 \) km/h. The still-water speed is always the average of the two given speeds, so it must fall between 5 and 11 - which alone rules out two of the options.
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