From the Super Mario Wiki
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The player starts next to a group of Springboards, with the Mini-Mario stationed on the far right. After collecting the Mini-Mario, the player needs to approach the Springboard in the center of the level from the left, then jump up three Springboards to reach the platform on the upper-right. Climbing up a ladder, the player can use a Hammer to defeat a Shy Guy and a couple of Oils, then move left so the Mini-Mario climbs into a passage. Dropping down just off of the platform will cause the player and the Mini-Mario to fall onto the platform with the locked door, allowing the player to clear the level.