One-third of a journey is covered at a speed of 21 km/h, the next one-third at 63 km/h, and the final one-third at 86 km/h. The average speed (in km/h), rounded off to 1 decimal place, for the entire journey is:

Correct option is C

Let total distance = 3x. Each part = x km. Time taken:
· x/21 + x/63 + x/86 = (86×63 + 21×86 + 21×63)/(21×63×86) = total time L.C.M. calculation gives total time ≈ 3x / 39.9 Average speed = Total Distance / Total Time = 3x / (3x/39.9) = 39.9 km/h
Explanation of options: (a) ✘ 41.6 is slightly high due to wrong harmonic mean. (b) ✘ 38.6 is close but not rounded accurately. (c) ✔ Harmonic mean gives exact average for equal distances → 39.9 km/h. (d) ✘ 43.4 is too high and doesn’t match the calculation.