It takes a ship 3 hours to cover 72 km with the current and 4 hours against the current. find the speed of the ship in still water and the speed of the current.

X- speed of the ship in still water
a- speed of current
(x+a)- speed with current
(x-a) - speed against current

(x+a)*3=72
(x-a)*4=72

      x+a=24
+    x-a=18
    2x    =42
      x = 21

x+a=24,    21+a=24,    a=3

The speed of the ship in still water = 21 km/h, 
the  speed of the current =3 km/h.