The Fly and the Bicycle

Here is a classic problem. Little Darius and little Xerxes were 10 miles apart. They rode their bicycles at a constant 10 miles/hr toward each other. As they started off, a fly took off from one bicycle and flew to the other bicycle at 20 miles/hr, then it flew back to the other bicycle, and then back and forth until it was crushed between the two bicycles when the two bicycles collided. Pretty stupid, all around. How far did the fly fly?