Discussion
In one hour, a boat goes 11km along the stream and 5 km against it. Find the speed of the boat in still water
Answer: C
With \( b \) the still-water speed and \( r \) the stream's,
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b = \frac{(b+r) + (b-r)}{2}, \qquad r = \frac{(b+r) - (b-r)}{2} \]
\[ b = \frac{11 + 5}{2} = 8 \text{ km/hr} \]
The current runs at \( \tfrac{11-5}{2} = 3 \) km/hr. Since the still-water speed is the average, it must fall between 5 and 11 - which eliminates 6 as too close to the slow end without any calculation.
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