Average speed over two legs — Averages Aptitude Question
Average speed over two legs: A car covers 100 km at 60 km/h and another 100 km at 40 km/h. Answer and worked solution on CodeKairo.
- Topic: Averages
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 75 seconds
Question
A car covers 100 km at 60 km/h and another 100 km at 40 km/h. What is its average speed for the whole journey?
Options
- 45 km/h
- 48 km/h
- 50 km/h
- 52 km/h
Show the answer and the worked solution
Correct answer: B. 48 km/h
Worked solution
- Time = 100/60 + 100/40 = 5/3 + 5/2 = 25/6 hours.
- Total distance = 200 km.
- Average speed = 200 ÷ (25/6) = 48 km/h.
Answer: 48 km/h.
The idea
For equal distances the average speed is the harmonic mean 2ab/(a + b) = 48, always below the arithmetic mean of 50. Equal times would give 50 instead.
Shortcut. Pick a leg length both speeds divide, say 120 km: 2 h + 3 h = 5 h for 240 km, so 48 km/h — the answer does not depend on the distance.
Common trap. "50 km/h" — averaging the two speeds; the car spends longer at 40 km/h, which pulls the average below 50.