Q.Write the set $\{5, 10, 15, 20, 25\}$ in set-builder form.

A set is a well-defined collection of distinct objects (its elements), written using curly-brace notation, with membership shown by $\in$ and non-membership by $\notin$. A set can always be represented in either of two equivalent ways: roster form, which lists every element explicitly (e.g. $\{2, 4, 6, 8, 10\}$), or set-builder form, which states the defining property every element must satisfy (e.g. $\{x : x \in \mathbb{N}, x \text{ is even}, x < 11\}$). Neither form changes what the set contains — they are two notations for the same underlying collection, and converting fluently between them (in both directions) is the core skill this concept tests.