Multiple choice

A car covers 3 successive 4 km distances at a speed of 10 kmph, 20 kmph and 60 kmph, respectively. Its average speed over total distance is

  1. 18 kmph

  2. 20 kmph

  3. 25 kmph

  4. 30 kmph
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A Correct answer
Explanation

Average speed = Total Distance / Total Time. Total distance = 4 + 4 + 4 = 12 km. Total time = (4/10) + (4/20) + (4/60) = 0.4 + 0.2 + 0.0666 = 0.6666 hours. Average speed = 12 / (2/3) = 18 kmph.

AI explanation

To find the average speed for equal distances, use the harmonic mean formula: average speed equals 3 divided by the sum of the reciprocals of the individual speeds. The time taken to travel the first 4 km is 4 divided by 10 hours, the second 4 km is 4 divided by 20 hours, and the third 4 km is 4 divided by 60 hours. The total distance of 12 km divided by the total time of (0.4 plus 0.2 plus 0.066) hours gives the average speed. The total time is 12 divided by 60 plus 6 divided by 60 plus 2 divided by 60, equaling 20 divided by 60 or 1 divided by 3 hours, making the average speed 12 divided by 1 divided by 3, which equals 18 kmph.

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