A man whose speed is 4.5 kmph in still water rows to a certain upstream point and back to the starting point in a river which flows at 1.5 kmph, find his average speed for the total journey?
Correct answer: B. 4 kmph
- A. 8 kmph
- B. 4 kmph
- C. 2 kmph
- D. 10 kmph
Explanation
The upstream speed is 4.5 km/hr - 1.5 km/hr = 3 km/hr, and the downstream speed is 4.5 km/hr + 1.5 km/hr = 6 km/hr. For equal distances, average speed is total distance divided by total time, or the harmonic mean: 2 x 3 km/hr x 6 km/hr ÷ (3 km/hr + 6 km/hr) = 4 km/hr. The likely mistake is option a, 8 km/hr, which treats the average as an ordinary average or ignores the slower upstream leg.
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