The distance between the two berths A and B on the river the raft sails in 18 hours

The distance between the two berths A and B on the river the raft sails in 18 hours, and the boat the same distance against the river flow in 2 hours. How many hours does the boat sail the distance AB across the lake?

Let’s designate: x km – distance between two berths.
Let’s find the speed of the raft (the speed of the river flow):
x / 18 km / h;

Let’s find the speed of the boat against the current:
x / 2 km / h;

Let’s find the boat’s own speed:
x / 2 + x / 18 = 9x / 18 + x / 18 = 10x / 18 km / h;

The boat sails along the lake at its own speed. Let’s find the time of movement of the boat on the lake:
x / (10x / 18) = 18x / 10x = 1.8 hours;

Answer: the boat sails along the lake between the berths in 1.8 hours.