Two cars start simultaneously towards each other from two cities located at the distance of 100 mi. One car goes at the speed of 20 miles per hour, the other at 30 mph. A fly was sitting on one of the cars before the race started. Startled by a revving engine, the fly alights from one car and flies towards the other at the speed of 40 mph. The moment it reaches the other car, it turns back and proceeds at the same speed and turns back again just a moment before it is squashed by the incoming car. Thus it flies back and forth until it meets its sad fate when the cars bump into each other. How big the distance has been covered by the fly before its untimely end?