Harry, Jim and Tom compete in a 100m race. They all run at a constant speeds (well, they are mathematical beings, not real runners!).
Harry beat Jim by 10m, Jim beat Tom by 10m. By how much does Harry beat Tom?
Solution: The obvious answer that Harry beat Tom by 20m is, of course, the wrong answer!
Harry beat Jim by 10m means Harry covers 100m in the same time that Jim covers 90m, so Jim's speed is only 9/10 of Harry's. Jim beats Tom by 10m means that Jim covers 100m in the same time that Tom does 90m, so Tom's speed is only 9/10 of Jim's.
So Tom's speed is 9/10 of 9/10 of Harry's. That is Tom's speed is only 81/100 of Harry's, so when Harry finishes 100m Tom is only at the 81m mark.