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Question

A man rowing at the rate of 5 km an hour in still water takes thrice as much time in going 40 km up the river as in going 40 km down.Find the rate at which the river flows.

A
12 km/hr
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B
9.5 km/hr
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C
2.5 km/hr
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D
15 km/hr
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Solution

The correct option is C 2.5 km/hr
Lets assume that river flows at V km/hr.
Upstream velocity =(5V) km/hr
Downstream velocity =(5+V) km/hr
As he travels equal distance in both directions.
Tup(5V) = Tdown(5+V), where Tup and Tdown are time taken for upstream and downstream travelling respectively.
It is given that Tup=3Tdown
Therefore, 3(5V)=5+V
V=2.5 km/hr

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