Multiple choice

A bus travelled the first one-third distance at a speed of 10 km hr-1, the next one-third at 20 km hr-1 and the remaining one-third at 60 km hr-1. The average speed of the bus was

  1. 18 km hr-1

  2. 16 km hr-1

  3. 9 km hr-1

  4. 48 km hr-1
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A Correct answer
Explanation

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

AI explanation

When equal distances are travelled at different speeds, the average speed is the harmonic mean of the individual speeds. Assuming the total distance is 60 km for easy calculation, each one-third part equals 20 km, meaning the time taken for the three parts is 2 hours, 1 hour, and 1/3 of an hour respectively. The total time for the journey is 10/3 hours, making the average speed 60 km divided by 10/3 hours, which equals 18 km/hr.

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