A man takes twice as long to row up as to row down the river. If the rate of river is 4km/hr.find the rate of the man in still water

Speed of the boat in still water =9 km

Let D be the distance. It takes him twice the time to row upstream compared to downstream.

Let speed of the current be x kmph

Speed downstream =(9+x)

Speed upstream = (9-x)

Ratio of time taken for upstream to time taken for downstream = D/(9-x) ÷ D/(9+x) =1/2

2 (9 - x)= 9+x

18 -2x = 9+x

-3x =-9 hence x = 3

Rate of stream = 3 kmph.

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