Answer with 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.