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A man can row 30 km downstream and 20 km upstream in 4 hours. He can row 45 km downstream and 40 km upstream in 7 hours. Find the speed of man in still water?

Correct answer: D. 12.5 kmph

  • A. 15 kmph
  • B. 10 kmph
  • C. 12 kmph
  • D. 12.5 kmph
  • E. None of these

Explanation

Let the downstream and upstream speeds be x km/hr and y km/hr. The equations are 30 km/x km/hr + 20 km/y km/hr = 4 hours and 45 km/x km/hr + 40 km/y km/hr = 7 hours; solving gives x = 15 km/hr and y = 10 km/hr. Therefore, the still-water speed is (15 km/hr + 10 km/hr) ÷ 2 = 12.5 km/hr. The likely mistake is option b, 10 km/hr, which is the upstream speed rather than the still-water speed.

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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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