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Question

A motor boat going downstream crosses a float at a point A. 60 minutes later it turns back and after some time it again crosses the float, now the float is at a distance of 12 km from the point A. The velocity of the stream is

A
8 km/h
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B
4 km/h
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C
6 km/h
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D
10 km/h
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Solution

The correct option is D 6 km/h
Let the velocity of boat =V
Velocity of stream =v
Float is at a point A
Boat coves AB distance in 60min
Float covers AC distance in (t+60)min
Boat covers BC distance in tmin
so, v(60+t)=12 ..... (1)
(v+V)60=AB .... (2)
(Vv)t=BC=AB12 ... (3)
Solving equation (1), (2) and (3) we get t=60
So, v(120)=12
v=110km/min=6010km/h=6 km/h
86096_2698_ans.jpg

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