Brain Teaser!!!!?

2 boys on a bicycle, 20 miles apart, began racing towards each other. The instant they started a fly on the handle bar of one od the bikes started flying towards the other bike's handle bar. As soon as it reached, it turned around and went to the other bike's handle bar, and so on until the bikes met. If each bike has a constant speed of 10 mph and the fly was traveling 15 mph constantly, how far did he travel?