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Let ss be the minimum number of sets of spoons Sydney should buy.
The total number of spoons needed is 525 - 335 = <<525-335=190>>190.
Each set contains 20 spoons, so Sydney should buy at least ss sets.
Therefore, the inequality is ss >= 190/20.
Rewriting the inequality in a simplified form, we get ss >= <<190/20=9.5>>9.5. Answer: \boxed{ss \geq 9.5}.