Moving Boat

A boat sails on a river against its current from point $A$ to point $B$ and back to point $A$ again. The velocity of the boat in relation to water is identical in both cases and is equal to $\text{4 km/h}$ . The velocity of the current is $\text{1.6 km/h}$ . Determine the ratio of the time the boat takes to sail from point $A$ to point $B$ and back and the time it would take the boat to cover the same distance on a still lake.