word problem: fly by train

Two trains are on the same track 125 miles apart traveling toward each other, one at 55 mph and the other at 45 mph. A fly going 85 mph starts at one train, flies to the other train, immediately turns around and flies back to the first train. It continues going back and forth between the trains until they collide. How many miles did the fly travel?

GC4EVR

If one train is travelling at 55 mph and the other at 45 mph, then the distance between them is diminishing at a rate of 100 mph. So, if they are 125 miles apart when they start, it will take an hour and a quarter for them to collide.

The fly will therefore be travelling at 85 mph for a hour and a quarter, and will travel 106.25 miles.