There are two cities along the river, 360 kilometers apart. It takes 35 hours for ship A to travel back and forth between the two cities, of which sailing with the current is 5 hours less than sailing against the current, and the speed of ship B is 15 km/h. So how many hours does it take for B ship to go back and forth between the two cities? According to the topic, it takes 15 hours for Serie A ships to sail smoothly and 20 hours to sail against the current.

Then the speed of ship A is 360/ 15=24km/h downstream and 360/20= 18km/h downstream.

The speed of ship A in the downstream water = the speed of ship A+the speed of water = 24 km/h.

The speed of ship A in the current = the speed of ship A-the current speed =18km/h.

By adding the two formulas, it is simplified to ship speed A =2 1km/h and water speed = 3 km/h.

If the speed of ship B is 15km/h, then

The downstream speed of B ship = B ship speed+current speed =18km/h.

The speed of ship B in the current = the speed of ship B-the current speed =12 km/h.

It takes 360/ 18=20 hours for ship B to run smoothly.

It takes 360/ 12=30 hours for ship B to go upstream.

So it takes 50 hours for B boat to go back and forth.