Multiple choice

A trader covers four successive 6 km stretches at speeds of 25 kmph , 50 kmph , 75 kmph , and 150 kmph respectively. Its average speed over this distance is

  1. 25 kmph

  2. 50 kmph

  3. 75 kmph

  4. 150 kmph
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Average speed = total distance / total time. Total distance = 24 km. Total time = 6/25 + 6/50 + 6/75 + 6/150 = 6(1/25+1/50+1/75+1/150) = 6(6/150+3/150+2/150+1/150) = 6(12/150) = 72/150 hr. Speed = 24/(72/150) = 50 km/h. This is the harmonic mean of 25, 50, 75, 150 for equal distances. Option C (75) is a distractor - the arithmetic mean of extremes.

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