2015 · Q21
A car travels the first one-third of a certain distance with a speed of 10 km/hr, the next one-third distance with a speed of 20 km/hr and the last one-third distance with a speed of 60 km/hr. The average speed of the car for the whole journey is
Elementary MathematicsTime, Speed and Distance
- A18 km/hr
- B24 km/hr
- C30 km/hr
- D36 km/hr
Explanation
This tests average speed over equal distances — the harmonic mean, not the arithmetic mean. Average speed = total distance / total time = 3/(1/10 + 1/20 + 1/60) = 3/(10/60) = 18 km/h. Option (c) 30 km/h is the trap from averaging the three speeds directly; because more time is spent at the lower speeds, the true average is pulled down. Hence option (a).