Multiple choice

A car moving on a straight road covers one third of a certain distance with 20 Km/h and the rest with 60 Km/h. The average speed is :

  1. 40 km/h

  2. 50 km/h

  3. 36 km/h

  4. 55 km/h
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let total distance be 3D. Time taken = (D/20) + (2D/60) = (3D + 2D) / 60 = 5D/60 = D/12. Average speed = Total distance / Total time = 3D / (D/12) = 36 km/h.

AI explanation

Using the harmonic mean formula for average speed when equal distances are travelled at different speeds, we calculate 2xy divided by (x + y). Let the total distance be 3x, so one third is x and the rest is 2x. The average speed is the total distance 3x divided by the sum of the times (x/20 + 2x/60), which simplifies to 3x divided by (5x/60), resulting in 36 km/h.

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