Vida,

The key fact here is that rate = distance/time or said another way distance = time × rate.

Suppose that the speed of the water current is w km/hr and the speed of the boat in still water is b km/hr.

On the trip upstream the boat is pushing ahead at b km/hr but being pushed back by the stream at w km/hr and hence the net speed of the boat is (b - w) km/hr. At this speed it travels 20 km in 6 hours and distance = time × rate so

20 = 6 × (b - w)

Write a similar equation for the trip downstream. Now you have two equation in b and w which you can solve.

I hope this helps,
Penny