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A boat is rowed downstream at 15.5km/hr and upstream at 8.5 km/hr. The speed of the stream is________?

Correct answer: A. 3.5km/hr

  • A. 3.5km/hr
  • B. 5.75km/hr
  • C. 6.5km/hr
  • D. 7km/hr

Explanation

The difference between downstream and upstream speeds is caused by twice the stream speed. Thus, stream speed = (15.5 km/hr - 8.5 km/hr) ÷ 2 = 7 km/hr ÷ 2 = 3.5 km/hr. The likely mistake is option d, 7 km/hr, which is the total difference rather than half the difference.

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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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