Downstream and back — Aptitude Question
Downstream and back: A boat's speed in still water is 12 km/h and the stream flows at 3 km/h. Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 60 seconds
Question
A boat's speed in still water is 12 km/h and the stream flows at 3 km/h. How long does the boat take to go 45 km downstream and return?
Options
- 7 h
- 8 h
- 9 h
- 10 h
Show the answer and the worked solution
Correct answer: B. 8 h
Worked solution
- Downstream: 15 km/h, so 45/15 = 3 h.
- Upstream: 9 km/h, so 45/9 = 5 h.
- Total = 8 hours.
Answer: 8 h.
The idea
Downstream = boat + stream; upstream = boat − stream. Given both, boat speed is their average and stream speed is half their difference.
Shortcut. Round-trip time = 2db ÷ (b² − s²) = 2 × 45 × 12 ÷ (144 − 9) = 1,080 ÷ 135 = 8 h.
Common trap. 10 h — doubling the upstream leg (2 × 45/9) as if the boat fought the current both ways; downstream it takes only 3 h.