Average speed there and back — Aptitude Question
Average speed there and back: A man goes from A to B at 40 km/h and returns at 60 km/h. What is his average speed… Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 75 seconds
Question
A man goes from A to B at 40 km/h and returns at 60 km/h. What is his 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
- Take the distance as 120 km each way. Times: 120/40 = 3 h and 120/60 = 2 h.
- Total = 240 km in 5 h.
- Average = 48 km/h, matching 2 × 40 × 60/100.
Answer: 48 km/h.
The idea
Equal distances give the harmonic mean 2ab/(a + b), always below the arithmetic mean. Equal times would give the arithmetic mean of 50 instead.
Shortcut. More time is spent at the slower 40, so the average falls below 50 — striking out 50 and 52 — and 2 × 40 × 60 ÷ 100 = 48 picks it from 45.
Common trap. 50 km/h — the plain mean of 40 and 60, which would need equal times; with equal distances the slower leg lasts longer and drags the average down.