Solution for Crossing the River ...

The sequence of crossings is as follows:

1.  The two sons cross.

2.  One of the sons rows back

3.  The first man rows across

4.  The other son rows back.

5.  Both sons row across again

6.  One of the sons rows back

7.  The second man rows across

8.  The other son rows back.

9.  Both sons row cross again - and everyone has crossed.