A boat covers 35 km downstream in 2 h and covers the same distance upstream in 7 h. Find the speed (in km/h) of the boat in still water.
- AOption (a)
11.25
- BOption (b)
28.75
- COption (c)
18.25
- DOption (d)
10.55
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Correct answer
11.25
Why this answer is correct
Shortcut Trick
Downstream Speed (D) = Distance ÷ Time = 35 ÷ 2 = 17.5 km/h
Upstream Speed (U) = Distance ÷ Time = 35 ÷ 7 = 5 km/h
Speed in Still Water = (D + U) ÷ 2
Speed = (17.5 + 5) ÷ 2 = 22.5 ÷ 2 = 11.25 km/h
∴ The correct answer is 11.25 km/h.
Alternate Method
Given:
Distance (d) = 35 km
Time taken downstream (td) = 2 h
Time taken upstream (tu) = 7 h
Formula Used:
Speed = Distance ÷ Time
Downstream speed = u + v
Upstream speed = u − v
(where u = speed of boat in still water and v = speed of stream)
Calculations:
⇒ Downstream speed (u + v) = 35 ÷ 2 = 17.5 km/h ---- (1)
⇒ Upstream speed (u − v) = 35 ÷ 7 = 5 km/h ---- (2)
⇒ Adding equations (1) and (2):
⇒ (u + v) + (u − v) = 17.5 + 5
⇒ 2u = 22.5
⇒ u = 22.5 ÷ 2
⇒ u = 11.25 km/h
∴ The correct answer is 11.25 km/h.
Additional Information
Speed of Stream
The speed of the water current or stream (v) is calculated as: v = (Downstream Speed − Upstream Speed) ÷ 2.
Relative Speed in Boats
When moving downstream, the speeds are added (u+v); when moving upstream, the stream speed is subtracted from the boat speed (u−v).
Ratio of Speeds
If the distance is constant, the ratio of downstream speed to upstream speed is the inverse of the ratio of time taken: D/U = Tu/Td.







































