A man can swim in still water at a speed of 6 km/h, but he takes thrice as long to swim upstream as downstream. The speed of the man, when swimming downstream is :

9 km/h

Let the speed of the river =  a km/h

Speed of man during upstream = 6 - a

Speed of man during downstream = 6 + a

Let the distance be X km

Time taken in the upstream = 3 × (Time taken in the downstream)

\(\frac{X }{6 - a }\)  = \(\frac{3X }{6 +a }\) 

3(6-x) = (6 + x )

18 - 3x = 6 + x

12 = 4x

x = 3 km/h

So speed of man in downstream = 6 + 3 = 9 km/h