From the Super Mario Wiki
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The player first has to climb a ladder to activate the Mini-Mario, and then press the blue Color Switch. When the Snapjaw is below the platform the player is standing on, the player and the Mini-Mario need to drop down and head for the ropes to the left, then time dropping a fruit onto the Snapjaw as it approaches. The player then has to jump onto the platform to the left and drop down, then move right to reunite with the Mini-Mario. Next, the player has to defeat two more Snapjaws as the player moves right, so they do not attack the Mini-Mario. Continuing right, the player and the Mini-Mario have to use a Springboard to reach the locked door.