Explanation:

 Speed Ratio Downstream : Upstream 

                                     6   :   2

                                => 3  :   1

Since the distance is same for both the cases, 

Time Ratio for these two is :-

                                     1   :   3

Total time is 4 hrs, So Time taken in these cases are 1 hr and 3 hrs respectively.

So distance can be found by any of the case.

D = S * T = 6km/hr * 1 hr = 6 km.