Bicycles and Fly Puzzle

This puzzle seems harder than it is at first.

Many people start by trying to add up the entire series of the fly’s back-and-forth trips. But there’s a much simpler way: all you need to figure out is how long it takes the bicycles to meet. Once you know that, calculating the fly’s distance travelled is easy.

Some puzzles have what puzzle-writer Martin Gardner called an “aha!” moment. This is when something clicks and you suddenly see that the solution is much simpler than it first appeared.