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Question

A boat having a speed of 5 km/h in still water, crosses a river of width 1 km along the shortest possible path in 15 min. The speed of the river water in km/h is

A
1
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B
3
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C
4
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D
41
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Solution

The correct option is B 3
The velocity of the boat in still water=5km/h
To cross the river in the shortest possible path it has to be done perpendicular to the flow of the river.
Velocity of boat in water moving perpendicular to flow =1/0.25=4km/h
Let v be the velocity of the flow of the river.
52=42+v2+2×4×v cos 90ov2=2516=9v=3 km/hr

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