Two ferry boats ply the same route between ports on opposite sides of
a river. They set out simultaneously from opposite ports, but one is
faster than the other, so they meet at a point 720 yards from the
nearest shore. When each boat reaches its destination, it waits 10
minutes to change passengers then begins its return trip. Now the boats
meet at a point 400 yards from the other shore. How wide is the river?



Note: 1760 yards equals 1 mile.

The source code of a Python program which simulates the progress of the two
ferries is at http://primepuzzle.com/tc/ferry.py

Note: The fraction in the assignment statement

fp = 13.0/9.0*t

in the Python program (see above link) comes from

f*t/s*t = (d-720)/720 = (1760-720)/720 = 1040/720 = 13/9

https://youtu.be/PQZUXeFDc-Y