A man can row at 12 km/hr in still water. He finds that he takes twice as much time to row upstream than to row downstream when he covers a certain distance. Find the speed of the stream.

As per the question, he takes twice the time when he rows upstream than rowing downstream.

Time is inversely proportioned to speed. So he rows at twice the speed when moving along the stream than moving against the stream.

Let his speed upstream = X km/hr

And, his speed downstream = 2X

∴ Speed in still water = 1/2 (2X+X) = 1.5 X km/hr

As per the question;

1.5X = 12 km/hr

X = 120/15 =

8 km/hr

Speed upstream = 8 km/hr

So, speed downstream = 2 ∗ 8= 16 km/hr Speed of current = 1/2 (speed downstream – speed upstream)

= 1/2 (16-8)

= 1/2 * 8 = 4 km/hr