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

$B-C$

Show that if $A \subset B,$ then $(C-B) \subset( C-A)$

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

$A-D$

Find the union of each of the following pairs of sets :

$X =\{1,3,5\} \quad Y =\{1,2,3\}$

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

Find the union of each of the following pairs of sets :

$A=\{a, e, i, o, u\} B=\{a, b, c\}$