The distance between the two ports is 75km, and it takes 20 hours for a passenger ship to travel back and forth between the two ports. It is known that the downstream speed is three times that of the countercurrent, and the still water speed of a cargo ship is twice that of a passenger ship. So how many hours does it take for a cargo ship to go back and forth between two ports?

The downstream velocity is three times that of the countercurrent.

Then the ratio of downstream sailing time to upstream sailing time is 1: 3.

So the downstream sailing time =20× 1/( 1+3)=5 hours.

Countercurrent sailing time =20-5= 15 hours

Downstream speed of passenger ship = 75/5 = 15km/h

Countercurrent velocity = 75/15 = 5 km/h.

Hydrostatic velocity = (15+5)/2 =10 km/h.

Water velocity = (15-5)/2 = 5 km/h.

The still water speed of the cargo ship =10× 2 = 20km/h.

It takes 75/(20+5)+75/(20-5)=3+5=8 hours for a cargo ship to travel to and from two ports.