Still-water speed from two times — Aptitude Question
Still-water speed from two times: A man rows 30 km downstream in 2 hours and the same distance upstream in 6 hours. Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 60 seconds
Question
A man rows 30 km downstream in 2 hours and the same distance upstream in 6 hours. What is his speed in still water?
Options
- 8 km/h
- 10 km/h
- 12 km/h
- 15 km/h
Show the answer and the worked solution
Correct answer: B. 10 km/h
Worked solution
- Downstream = 15 km/h; upstream = 5 km/h.
- Still water = (15 + 5)/2 = 10 km/h.
Answer: 10 km/h.
The idea
Convert both legs to speeds before averaging. Averaging the times instead is meaningless.
Shortcut. Times 2 : 6 mean speeds 3 : 1, so boat : stream = (3 + 1) : (3 − 1) = 2 : 1; downstream 15 is 3 parts, so the boat's 2 parts are 10 km/h.
Common trap. 15 km/h — reading the downstream speed (30 ÷ 2) as the still-water speed; the stream is helping on that leg.