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Question

A man can row a boat with 4km/h in still water. If he is crossing a river where the current is 2km/h and width of river is 4km. How long will it take him to row 2km up the stream and then back to his starting point?

A
2hrs
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B
1hr
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C
43hrs
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D
None of the above
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Solution

The correct option is C 43hrs
Given :
Velocity of man wrt water, VMW=4 km/hr
Velocity of river, VR=2 km/hr
Distance covered in upstream or downward motion d=2 km each

For upstream motion of the boat :

Velocity of the man , VM=VMWVW=42=2 km/h
Time taken to go upstream t1=22=1 hr

For downward motion of the boat or (return journey) :

Velocity of the man VM=VMW+VW=4+2=6 km/h
Time taken to go downstream t2=26=13 hr

Total time taken T=t1+t2=1+13=43 hrs

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