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Question

# A man can swim in still water with a velocity 5m/s. He wants to reach at directly opposite point on the other bank of a river which is flowing at a rate of 4m/s. River is 15m wide and the man can run with twice the velocity as compared with velocity of swimming. If he swims perpendicular to river flow and then run along the bank, then time taken by him to reach the opposite point is:

A
3sec
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B
less than 3sec
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C
5sec
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D
4.2sec
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Solution

## The correct option is D 4.2secVelocity of swimming of man in still water VmR=5m/sTime taken in crossing the river T1=dVmR∴ T1=155⟹T1=3secVelocity of flow of water VR=4m/sThe flow of water helps the man to reach point B which is at a distance D from point P.Thus Distance traveled in this time along the river D=VR×T1∴D=4×3⟹D=12mVelocity with which the man can run V′=2VmRTime taken by man to reach P from B by running T2=D2VmR ∴ T2=122×5=1.2secTotal time taken by man T=T1+T2=3+1.2=4.2sec

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