Discussion
A boat whose speed in still water is 10 km/hr travels 30 km downstream and returns, the stream running at 5 km/hr. What is its average speed for the whole journey ?
Answer: B
Correct answer: 7.5 km/hr
\[\text{Downstream time} = \frac{30}{15} = 2 \text{ h}, \qquad \text{Upstream time} = \frac{30}{5} = 6 \text{ h}\]
\[\text{Average speed} = \frac{\text{total distance}}{\text{total time}} = \frac{60}{8} = 7.5 \text{ km/hr}\]
Note that this is well below the still-water speed of 10 km/hr: the slow return leg occupies most of the time, so it dominates the average.
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