Q.If $A = \{2, 4, 6, 8, 10\}$ and $B = \{3, 6, 9, 12\}$, find $A \cup B$ and $A \cap B$.

[!TLDR] Combine and intersect two small numeric sets. [!ANSWER] $A \cup B = \{2,3,4,6,8,9,10,12\}$, $A \cap B = \{6\}$.

$A = \{2,4,6,8,10\}$, $B = \{3,6,9,12\}$. Union: list every element appearing in $A$ or $B$ (each once): $\{2,3,4,6,8,9,10,12\}$. Intersection: the only number appearing in both lists is 6 (since $6 \in A$ and $6 \in B$, while no other number is shared). [!ANSWER] $A \cup B = \{2,3,4,6,8,9,10,12\}$, $A \cap B = \{6\}$.

Merge both lists without repetition for the union; scan for common elements for the intersection.

A common slip is listing 6 twice in the union (once from each set) instead of once, since a set never repeats an element.