Solution: It takes 35 hours for a ship to go back and forth between two ports, and sailing against the current takes 5 hours more than sailing against the current.

Then it takes 15 hours for the downstream and 20 hours for the upstream.

Downstream speed 360/ 15=24 km/h

The countercurrent velocity is 360/20 =18 km/h.

Water velocity (24-18)/2 = 3 km/h.

At present, the speed of small cargo ships in still water is per hour 12 km.

The downstream speed is 3+12 =15 km/h.

Countercurrent velocity 12-3=9 km/h

It takes 360/ 15+360/9=24+40=64 hours for this small cargo ship to go back and forth between the two docks.

A: It takes 64 hours for this small freighter to go back and forth between the two docks.