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

Let $A = \{1, 2, 3, 4, 5, 6\}$ and $B = \{2, 4, 6, 8\}$. Find $A - B$ and $B - A$.

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

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 $A - B$.

$A$ class has $175$ students. The following data shows the number of students opting for one or more subjects: Mathematics $100$,Physics $70$,Chemistry $40$; Mathematics and Physics $30$,Mathematics and Chemistry $28$,Physics and Chemistry $23$; Mathematics,Physics,and Chemistry $18$. How many students have opted for Mathematics alone?

If $A = \{3, 5, 7, 9, 11\}$,$B = \{7, 9, 11, 13\}$,$C = \{11, 13, 15\}$,and $D = \{15, 17\}$,find $B \cap D$.