The speed of the boat along the river is 18.2 km / h, and 12.6 km / h upstream. How long will the boat take 23.1 km along the lake.

First, we need to know the speed of the river, for this we need the speed of the boat downstream, subtract the speed of the boat upstream and divide the resulting difference by 2:

1) (18.2 – 12.6) / 2 = 5.6 / 2 = 2.8 kilometers per hour.
Now we can find out the speed of the boat in still water:

2) 18.2 – 2.8 = 15.4 kilometers per hour.

And now you can calculate how long the boat will sail 23.1 kilometers in still water:

3) 23.1 / 15.4 = 1.5 hours = 1 hour 30 minutes.
Answer: the boat will sail 23.1 kilometers in 1 hour and 30 minutes.