Correct Answer: Option B
Let the speed of the stream be x km/hr then,
Speed downstream = (15+x) km/hr,
Speed upstream = (15 - x) km/hr.
So, 30/(15+x)+30/(15-x) = 4(2/1)
=> 900/(225-x2) = 9/2
=> 9x2 = 225
=> x2 = 25
=> x = 5 km/hr.
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