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
$D-C$
If $A=\{1,2,3,4\}, B=\{3,4,5,6\}, C=\{5,6,7,8\}$ and $D=\{7,8,9,10\} ;$ find
$A \cup B \cup C$
$A-(A-B)$ is
If ${N_a} = [an:n \in N\} ,$ then ${N_5} \cap {N_7} = $
Consider the sets $X$ and $Y$ of $X = \{ $ Ram , Geeta, Akbar $\} $ and $Y = \{ $ Geeta, David, Ashok $\} $ Find $X \cap Y$