World 2 - Level 5
This level presents a series of boulder puzzles. In the first room, push the top boulder along the purple path, then the bottom boulder along the red path to open the blue door.
In the second room, build a bridge using the two boulders along the purple path. Then activate the purple button by pushing the boulder along the red path. To collect the bonus coins later, you'll need to also move the top boulder along the yellow path followed by the next lower boulder along the white path. This will re-close the blue door, and hence disable the cannon guarding the Bonus Coins.
In the third room, do not collect the Rainbow Coins on the left! Use them as stoppers first, and push the top, then the bottom boulder along the purple paths.
In the fourth room, start along the purple paths, then push the boulder onto the green button along the red path. Finally, quickly (!) push the left boulder along the yellow path (this will disable the conveyor belt for a short period) and quickly (!) move the right boulder along the yellow path to build a bridge. Collect Bonus Coins and exit.