Two train engines are approaching each other on the same track.  They begin at a distance of 20 miles, and are each traveling at a rate of 10 mph.  At the instant they begin, a dragonfly takes off from the front of one train at a rate of 15mph.  When it meets the front of the other train, it immediately turns around and heads back towards the first train.  It continues back and forth until it is crushed when the two trains meet.

Q: What is the total distance traveled by the fly on its zig-zag journey before it meets its end?

Deadline : 7hours