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A man rows his boat 85 km downstream and 45 km upstream, taking 2 1/2 hours each time. Find the speed of the stream?

Correct answer: D. 8 kmph

  • A. 5 kmph
  • B. 6 kmph
  • C. 7 kmph
  • D. 8 kmph

Explanation

The downstream speed is 85 km ÷ 2.5 hours = 34 km/hr, and the upstream speed is 45 km ÷ 2.5 hours = 18 km/hr. The stream speed is half their difference: (34 km/hr - 18 km/hr) ÷ 2 = 8 km/hr. The likely mistake is option c, 7 km/hr, which does not account for the full difference between the two travel speeds.

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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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