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The time taken by a man to row his boat upstream is twice the time taken by him to row the same distance downstream. If the speed of the boat in still water is 42 kmph, find the speed of the stream?

Correct answer: C. 14 kmph

  • A. 12 kmph
  • B. 13 kmph
  • C. 14 kmph
  • D. 15 kmph

Explanation

For the same distance, time is inversely proportional to speed. The condition gives 1/(42 km/hr - s) = 2/(42 km/hr + s), so 42 km/hr + s = 84 km/hr - 2s and 3s = 42 km/hr, giving s = 14 km/hr. The likely mistake is option d, 15 km/hr, which comes from mishandling the factor of two in the time relationship.

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About Time, Speed and Distance

Problems relate distance, speed and time through the formula distance = speed × time, including unit conversion, average speed, relative speed and motion in opposite or the same direction. Questions may involve trains, boats and streams, where the effective speed changes, so average speed must not be confused with the simple average of two speeds.

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