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A person can row at 9 kmph and still water. He takes 4 1/2 hours to row from A to B and back. What is the distance between A and B if the speed of the stream is 1 kmph?

Correct answer: E. None of these

  • A. 32 km
  • B. 25 km
  • C. 28 km
  • D. 24 km
  • E. None of these

Explanation

The downstream speed is 9 km/hr + 1 km/hr = 10 km/hr and the upstream speed is 9 km/hr - 1 km/hr = 8 km/hr. If the one-way distance is d km, then d/(10 km/hr) + d/(8 km/hr) = 4.5 hours, so d x 0.225 per km = 4.5 hours and d = 20 km. The likely mistake is option d, 24 km, which results from using an incorrect combined-time calculation.

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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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