Three Karts, Two Gaps

Three go-karts — Alpha, Bravo, and Charlie — race down a straight 100-meter test track, each holding a perfectly constant speed from start to finish. At the instant Alpha crosses the line, Bravo still has 20 meters left to cover. And in a separate observation, at the instant Bravo crosses the line, Charlie still has 20 meters to go. How far behind Alpha is Charlie when Alpha finishes?