The Two Trains Riddle

Puzzle: Two trains are 100 miles apart and move towards each other, each at 50 miles per hour. A fly starts from one train, flying at 75 miles per hour to the other, and turns around each time it reaches a train. How far does the fly travel before the trains collide?

Answer: The fly travels 75 miles. The trains meet in 1 hour, and in that time, the fly travels continuously at 75 miles per hour.