Example 1 · level 1
A boat on the Hooghly has a speed of 9 km/h in still water. The stream flows at 3 km/h. How many hours will it take to go 24 km upstream?
Show solution
- Speed = 9 − 3 = 6 km/h.
- Time = 24 ÷ 6 = 4 h.
Answer: 4 hours
SSC & BANK EXAMS: QUANTITATIVE APTITUDE · TIME, SPEED & DISTANCE
Boats and streams
Downstream = b + s, upstream = b − s. Boat speed = (down + up)/2, stream speed = (down − up)/2.
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Worked examples
Example 2 · level 2
A boat goes 76 km downstream on the Ganga in 4 hours and 21 km upstream in 3 hours. Find the speed of the boat in still water (in km/h).
Show solution
- Downstream = 19 km/h, upstream = 7 km/h.
- Boat = (19 + 7)/2 = 13 km/h.
Answer: 13 kmph
Example 3 · level 3
A motorboat whose speed in still water is 17 km/h goes 140 km downstream on the Brahmaputra and comes back in a total of 17 hours. Find the speed of the stream (in km/h).
Show solution
- 140/(17 + s) + 140/(17 − s) = 4760/(289 − s²) = 17.
- 289 − s² = 4760 ÷ 17 = 280.
- s² = 9, so s = 3 km/h.
- Check: 140/20 + 140/14 = 7 + 10 = 17 h.
Answer: 3 kmph