Q59
ArithmeticSpeed, Time & Distance
A body covers half the distance with a speed of 20 m/s and the other half with 30 m/s. The average speed of the body during the whole journey is
- a.0 m/s
- b.24 m/s
- c.25 m/s
- d.28 m/s
Answer: (B) 24 m/s
When equal distances are covered at speeds $v_1$ and $v_2$, the average speed is the harmonic mean $\frac{2v_1v_2}{v_1 + v_2} = \frac{2 \times 20 \times 30}{20 + 30} = 24$ m/s. Option C (25 m/s) is the simple average, which would be correct only if equal times, not equal distances, were spent at each speed; the true value is lower because the body spends longer on the slower half. Option A confuses average speed with average velocity in a round trip. Check with a distance of 120 m: time $= \frac{60}{20} + \frac{60}{30} = 5$ s, so speed $= \frac{120}{5} = 24$ m/s.