When the ship turns back, the distance between the kettle and the ship is equivalent to: the ship travels 20 minutes against the current+the kettle travels 20 minutes (the current is 20 minutes) = the ship stays in still water for 20 minutes.

In time, the speed difference between the ship and the kettle is equivalent to the speed of the ship-the speed of the kettle (current speed) = the still water speed of the ship.

So the time to catch the kettle is 20 minutes. That is, the kettle took 20× 2 = 40 minutes and was washed away for 2 kilometers.

So the speed of water flow is 2 ÷ 40/60 = 3 kilometers per hour.