If $A$ and $B$ are two sets such that $A \subset B$,then what is $A \cup B$?

If $A = \{3, 6, 9, 12, 15, 18, 21\}$,$B = \{4, 8, 12, 16, 20\}$,$C = \{2, 4, 6, 8, 10, 12, 14, 16\}$,and $D = \{5, 10, 15, 20\}$,find $D - A$.

Let $A$ and $B$ be two sets,then $(A \cup B)' \cup (A' \cap B)$ is equal to

Which Venn diagram represents the truth of the statement "No policeman is a thief"?

If $A = \{1, 2, 3\}$,$B = \{3, 4\}$,and $C = \{4, 5, 6\}$,then find $A \cup (B \cap C)$.

The simplified switching circuit for the following circuit is