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Question 7
A person, rowing at the rate of 5 km/h in still water, takes thrice as much time in going 40 km upstream as in going 40km downstream. Find the speed of the stream.

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Solution

Let the speed of the stream be v km/h.
The speed of a person rowing in still water = 5 km/h
The speed of a person rowing downstream = (5 + v)km/h [His speed + speed of the stream]
And the speed of a person rowing upstream = ( 5 – v ) km/h [His speed - speed of the stream, as rowing upstream is in the direction opposite to that of the flow of the stream]
Now, time taken by the person to cover 40 km downstream is
t1=405+vh [speed=distancetime]
and the time taken by the person to cover 40km upstream is
t2=405vh
By the given condition, t2=t1 ×3
405v=405+v×3
v=104= 2.5 km/h
Hence, the speed of the stream is 2.5 km/h.


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