A man can row 28 km/h in still water.  If the speed of current is 12 km/h, it takes 15 hours more in upstream than the downstream for the same distance.  The distance is?

400 km

Speed of  man = x, Speed of water = y

 x = 25       y = 12

downstream speed (S_D) = 28 + 12 = 40 km/h

upstream speed (S_U) = 28 - 12 = 16 km/h

S_D  : S_U = 40 : 16

                 5  :  2

time           2  :  5 ... (Diff. 3 → ×5 → 15)

                  ↓                                 

                ×5

                  ↓      

              10 hours

Distance = 40 × 10  = 400 km