TWO FERRYBOATS start at the same instant from opposite sides of a river,
traveling across the water on routes at right angles to the shores. Each travels at a
constant speed, but one is faster than the other. They pass at a point 720 yards
from the nearest shore. Both boats remain in their slips for 10 minutes before
starting back. On the return trips they meet 400 yards from the other shore.
How wide is the river?