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A boat covers a distance of 30 km in 2 ½ hours running downstream while returning it covers the same distance in 3 ¾ hours. What is the speed of the boat in still water?

Correct answer: E. None of these

  • A. 8km/hr
  • B. 12km/hr
  • C. 14 km/hr
  • D. 15 km/hr
  • E. None of these

Explanation

The downstream speed is 30 km ÷ 2.5 hours = 12 km/hr, while the upstream speed is 30 km ÷ 3.75 hours = 8 km/hr. The still-water speed is the average of these two speeds: (12 km/hr + 8 km/hr) ÷ 2 = 10 km/hr, which is not listed. The likely mistake is option b, 12 km/hr, which is only the downstream speed, not 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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