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

Roster form lists the elements directly; spot the common arithmetic pattern they share and state it as a rule.

All four numbers are multiples of 5, stopping at 20.

Step 1 — Study the elements. $A = \{5, 10, 15, 20\}$. Each is obtained by multiplying 5 by $1, 2, 3, 4$ respectively, so every element is a multiple of 5.

Step 2 — Find the bound. The largest element is 20; the next multiple, 25, is excluded. So the rule should cap at $x \le 20$.

Step 3 — Write the rule and verify. Testing $x = 5, 10, 15, 20$ against "multiple of 5, $x \le 20$" confirms all four pass, and 25 correctly fails.

Writing the bound as $x < 20$ instead of $x \le 20$ would wrongly exclude 20 itself — always check whether the largest given element should be included or excluded before fixing the inequality sign.