Answer: 100 miles
Step-by-step explanation: How much time does the bird spend in flight? If you represent the problem in terms of the amount of time the bird is flying, then a solution is fairly straightforward. The two trains are 50 miles apart, and are approaching each other at a relative speed of 50 mph (25 + 25), so it will take one hour for them to meet. If the bird spends one hour flying at 100 mph, then it will traverse 100 miles before the trains meet.
