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Question

# 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=405−vh By the given condition, t2=t1 ×3 ⇒ 405−v=405+v×3 ∴ v=104= 2.5 km/h Hence, the speed of the stream is 2.5 km/h.

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