The trains close at 100 mph, so they meet after two hours. The fly is going at 75 mph the whole time, so it covers 150 miles, however many times it turns round.
Two trains start 200 miles apart on the same line, heading towards each other at 50 mph each. A fly sets off from the front of one and flies back and forth between them at 75 mph until the trains meet. How far does the fly travel?
Give me a hintThe hint
Forget the fly for a moment. How long until the trains meet?