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Congrats! 30 balls is the correct answer... As you mention in your comment, there are 16 balls on the lowest level (a square of 4x4 = 16), followed by a square of 3x3 = 9, then 2x2 = 4 and then the single ball on top. Your SBD is in your wallet. Hope to see you play again soon.

of course, I love math problems and puzzles