Explanation:

Suppose he move 4 km downstream in x hours. Then,

Speed Downstream = (4/x) km/hr. 

Speed Upstream = (3/x) km/hr. 

Therefore, 48 / (4/x) + 48 / (3/x) = 14 or x = 1/2 . 

So, Speed Downstream = 8 km/hr,
       Speed Upstream = 6 km/hr.

Rate of the stream = 1/2 (8 - 6) km/hr = 1 km/hr. 

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