A boat that has a speed of 5km/hr in still water crosses a river of width 1km along the shortest possible path in 20 minutes. The velocity of the river water in (km/hr) is
A
1
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B
3
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C
4
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D
4.5
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Solution
The correct option is C4 Displacement =1km t=2060=13hr
Velocity along AB=ABt
i.e vbrcosθ=113=3km/hr ⇒cosθ=3vbr=35 ⇒sinθ=45
From triangle ABC, sinθ=vrvbr=vr5
(∵ Horizontal component of velocity of boat must become zero) ∴vr=4km/hr