A man can row a boat at the rate of 4 km/hour in still water. He takes thrice as much time in going 30 km upstream as in going 30 km downstream. Find the speed of the stream.
Speed of boat in still water = 4 km/h
Let the speed of stream be 'x'
Therefore,
Speed of the boat upstream = Speed of boat in still water - Speed of stream = 4 - x
Speed of the boat downstream = Speed of boat in still water + Speed of stream = 4 + x
Time taken to go upstream 
Time taken to go downstream 
Given, time taken in upstream is thrice as in downstream
⇒ 4 + x = 12 - 3x
⇒ 4x = 8
⇒ x = 2
i.e. the speed of stream = x is 2 km/hour.
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