Two bicyclists start twenty miles apart and head toward each other, each going at a steady rate of 10 m. p.h. at the same time, a fly that travels at a steady 15 m. p.h. starts from the front wheel of the southbound bicycle and flies to the front wheel of the northbound one, then turns around and flies to the front wheel of the southbound one again, and continues in this manner till he is crushed between the two front wheels. question: what total distance did the fly cover?