Upstream takes twice as long — Aptitude Question
Upstream takes twice as long: A man takes twice as long to row upstream as downstream. If the stream flows at 3… Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 90 seconds
Question
A man takes twice as long to row upstream as downstream. If the stream flows at 3 km/h, what is his speed in still water?
Options
- 6 km/h
- 9 km/h
- 12 km/h
- 15 km/h
Show the answer and the worked solution
Correct answer: B. 9 km/h
Worked solution
- Twice the time upstream means the downstream speed is twice the upstream speed.
- b + 3 = 2(b − 3) = 2b − 6 → b = 9 km/h.
- Check: downstream 12, upstream 6 — indeed a 2 : 1 ratio. ✓
Answer: 9 km/h.
The idea
Convert the ratio of times into the inverse ratio of speeds first. Setting up the equation on times directly works too but is more error-prone.
Shortcut. Speeds down : up = 2 : 1, so boat : stream = (2 + 1) : (2 − 1) = 3 : 1, and the boat does 3 × 3 = 9 km/h.
Common trap. 6 km/h — copying the 2 : 1 time ratio onto boat and stream (2 × 3); that gives speeds 9 and 3, a 3 : 1 ratio, not 2 : 1.