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 $A-D$.

If $A = \{ x : x \text{ is an integer and } x^2 - 9 = 0 \}$,$B = \{ x : x \text{ is a natural number and } 2 \leq x < 5 \}$,and $C = \{ x : x \text{ is a prime number } \leq 4 \}$,then $(B - C) \cup A$ is:

If $A$ and $B$ are disjoint,then $n(A \cup B)$ is equal to

Find the union of each of the following pairs of sets:
$A = \{ x : x \text{ is a natural number and a multiple of } 3 \}$
$B = \{ x : x \text{ is a natural number less than } 6 \}$

Let $X = \{ \text{Ram, Geeta, Akbar} \}$ be the set of students of Class $XI$ who are in the school hockey team. Let $Y = \{ \text{Geeta, David, Ashok} \}$ be the set of students from Class $XI$ who are in the school football team. Find $X \cup Y$ and interpret the set.

If $X = \{4^n - 3n - 1 : n \in N\}$ and $Y = \{9(n - 1) : n \in N\}$,then $X \cap Y =$