The boat travels the distance between the two piers along the river in 2

The boat travels the distance between the two piers along the river in 2 hours, and the raft – in 8 hours. How long will the boat take on the way back?

Let us designate the distance between the wharves as A. Then the speed of the vessel along the river, if it is known that the time was 2 hours, will be equal to:

A / 2.

If it is known that the raft has been moving for 8 hours, then its speed was:

A / 8.

Because the boat initially moved downstream, so its own speed will be:

A / 2 – A / 8 = (4A – A) / 8 = 3A / 8.

Accordingly, when moving against the current, the speed will be:

3A / 8 – A / 8 = 2A / 8 = A / 4.

The movement time will be equal to: A: A / 4 = 4 (h).

Answer: 4 hours.