Two cyclist,20 miles apart,start at the same instant and ride towards each other along a straight road at a speed of 10 miles per hour. at the same instant a fly on the forehead of one of the riders starts to fly at 15 mile per hour toward the other rider, alights on his forehead, and the immediately flies back to the first rider. the fly travels back and forth over the continuously decreasing distance between the two riders until the two riders meet. how far has the fly flown when all its journeys are added together?

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