You can put this solution on YOUR website!
Let x and y be the speed of fish in still water and water current respectively.
Then the speed of fish
(1)when it swims downstream,
(2)when it swims upstream,
so by solving the above pair of linear equation we get x=4 & y=1.
So, the speed of the fish in still water is 4km/h & the speed of the current is 1km/h.