If $A$ and $B$ are two sets such that $A \subset B$, then what is $A \cup B ?$
If $A=\{3,5,7,9,11\}, B=\{7,9,11,13\}, C=\{11,13,15\}$ and $D=\{15,17\} ;$ find
$A \cap C$
If $A$ and $B$ are not disjoint sets, then $n(A \cup B)$ is equal to
If $A=\{3,6,9,12,15,18,21\}, B=\{4,8,12,16,20\},$ $C=\{2,4,6,8,10,12,14,16\}, D=\{5,10,15,20\} ;$ find
$A-D$
Find the union of each of the following pairs of sets :
$A=\{1,2,3\}, B=\varnothing$