Two ferry boats start moving at the same instant from opposite sides of a river.
One boat is faster than the other so that they meet (pass each other that is, not bash into each other) at a point 720 yards from the nearest bank.
After arriving at their destinations, each boat remains ten minutes in the slip to change passengers; then it starts on its return trip.
The boats meet again at a point 400 yards from the other bank.
What is the exact width of the river ?
