SOLUTION: A motorboat travels 336 kilometers in 7 hours going upstream. It travels 504 kilometers going downstream in the same amount of time. What is the rate of the boat in still water and

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.