Discussion
A boat can travel with a speed of 13 km/hr in still water. If the speed of the stream is 4 km/hr, find the time taken by the boat to go 68 km downstream.
Answer: C
Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b + r = 13 + 4 = 17 \text{ km/h} \]
\[ t = \frac{68}{17} = 4 \text{ hours} \]
Using the still-water speed alone would give \( \tfrac{68}{13} = 5.2 \) hours - the stream is pushing, so the journey must be quicker than that.
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