Using that for any sets $\mathrm{A}$ and $\mathrm{B},$
$A \cup(A \cap B)=A$
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
$D-A$
If $A=\{3,5,7,9,11\}, B=\{7,9,11,13\}, C=\{11,13,15\}$ and $D=\{15,17\} ;$ find
$\left( {A \cup D} \right) \cap \left( {B \cup C} \right)$
If $A, B, C$ are three sets, then $A \cap (B \cup C)$ is equal to
If $A$ and $B$ are sets, then $A \cap (B -A)$ is