Discussion
Speed of river is 6 km/hr. Speed of a motorboat in still water is 30km/hr. How much distance can it cover downstream in 24minutes?
Answer: D
Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b + r = 30 + 6 = 36 \text{ km/h} \]
\[ 24 \text{ min} = \frac{24}{60} = 0.4 \text{ h} \]
\[ \text{distance} = 36 \times 0.4 = 14.4 \text{ km} \]
Converting the minutes to hours before multiplying is the whole trick; leaving 24 in place would give 864 km - which is why 864 m appears as a distractor.
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