If $A=\{1, 2, 3, 4\}, B=\{3, 4, 5, 6\}, C=\{5, 6, 7, 8\}$ and $D=\{7, 8, 9, 10\}$,find $A \cup C$.

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

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

If $A$ and $B$ are any two non-empty sets and $A$ is a proper subset of $B$. If $n(A) = 4$,then the minimum possible value of $n(A \Delta B)$ is (where $\Delta$ denotes the symmetric difference of set $A$ and set $B$).

Find the union of each of the following pairs of sets:
$X = \{1, 3, 5\}, Y = \{1, 2, 3\}$

Let $A = \{a, b, c\}, B = \{b, c, d\}, C = \{a, b, d, e\}$. Then $A \cap (B \cup C)$ is