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