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Question

A boat which has a speed 5 km/h in still water crosses a river of width 100 m along the shortest possible path in 1.5 min. The velocity of 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
4.1
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Solution

The correct option is B 3
For shortest path velocity diagram is as shown
Given velocity of boat in still water vbw=5 km/h
let vb be velocity of boat, vw be velocity of water
Now time taken for boat to cross the river is t=1.5 min=wvb=100vb
vb=1001.5 mmin

vb=1001.5×11000×60 km/h
vb=4 km/h

Using pythagoras theorem
vw=v2bwv2b=5242
vw=3 km/h

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