Multiple choice

A car travels the first one-third distance at a speed of $10\ km/hr$, the next one-third distance at $20\ km/hr$ and the last one-third distance at $60\ km/hr$. The average speed of the car is

  1. $14\ km/hr$
  2. $16\ km/hr$
  3. $18\ km/hr$
  4. $20\ km/hr$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Average speed = Total distance / Total time. Let total distance be 3d. Time taken = d/10 + d/20 + d/60 = (6d+3d+d)/60 = 10d/60 = d/6. Average speed = 3d / (d/6) = 18 km/hr.

AI explanation

Using the average speed formula for three equal distances covered at different speeds, the calculation is 3 divided by the sum of the reciprocals of the individual speeds. Assuming a total distance of 60 km for simpler arithmetic, the time taken for the first, second, and third parts are 6, 3, and 1 hours respectively. The total time for the entire journey is the sum of these times, which is 10 hours. The average speed is the total distance of 60 km divided by the total time of 10 hours, resulting in 18 km/hr.

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