Level 20 is the twentieth level of the Main Tunnel.
Oh wow, is that a spiral level?
For this level, we highly recommend using the Skater as you can clear the level in one jump. No, seriously. One jump. This is one of the shortest levels in the game, if not the shortest, and you can see the end of the level from the end of Level 19.
When you start, change gravity to the right. Then when you get to the crumbling tile, jump. Then immediately move over to the right.
But with the Runner, it's harder to do that as you have to move left in midair (changing gravity in midair). You can also do this: Jump, move right, and repeat. You may lose the crumbling tiles, but they're a chain of one-tile wide platforms, and minus them should cause no problems.
And now the tunnel changes color. It's turning green.