Two trains are traveling toward each other on the same track. Initially they are 120 miles apart. One is traveling 25 miles per hour, and the other is traveling 35 miles per hour. At the instant that they are 120 miles apart, a fly sitting on the front of the first train flies toward the second train at a speed of 40 miles per hour. The moment it reaches the second train, it reverses direction and flies back toward the first train. Again it reverses direction, and continues to fly back and forth between the two trains until the trains meet. How far does the fly fly?