The effective rate upstream is   = 48 km/h,  and it is the difference 
of the boat rate in still water and the rate of current

    u - v = 48  km/h    (1)

   
The effective rate downstream is   = 72 km/h,  and it is the sum 
of the boat rate in still water and the rate of current

    u + v = 72  km/h.   (2)



Adding equations (1) and (2), you find  u = 60 km/h  for the rate of the motorboat at still water.


Subtracting equations (1) and (2), you find  v = 12  km/h  for the rate of current.