A round trip on the river — Aptitude Question
A round trip on the river: A boat whose speed in still water is 10 km/h travels 24 km downstream and back on a… Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 60 seconds
Question
A boat whose speed in still water is 10 km/h travels 24 km downstream and back on a stream flowing at 2 km/h. How long does the whole trip take?
Options
- 4 h
- 5 h
- 6 h
- 7 h
Show the answer and the worked solution
Correct answer: B. 5 h
Worked solution
- Downstream = 12 km/h → 24/12 = 2 h.
- Upstream = 8 km/h → 24/8 = 3 h.
- Total = 5 hours.
Answer: 5 h.
The idea
Compute each leg separately and add. The upstream leg always takes longer, so the total exceeds twice the still-water time.
Shortcut. Round-trip time = 2db ÷ (b² − s²) = 2 × 24 × 10 ÷ (100 − 4) = 480 ÷ 96 = 5 h.
Common trap. 4 h — doubling the downstream leg (2 × 24/12) as if the return also ran at 12 km/h; upstream the boat makes only 8 km/h.