The way I solved it was to say that the bicycles meet in the middle because their velocities are equal. They travel this 20/2=10 miles with v=10mph in 1 hour. I

The way I solved it was to say that the bicycles meet in the middle because their velocities are equal. They travel this 20/2=10 miles with v=10mph in 1 hour. In this 1 hour, the fly will travel 15 miles, no matter how often it changes its direction.