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

$C-D$

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

If $A=\{3,6,9,12,15,18,21\}, B=\{4,8,12,16,20\},$ $C=\{2,4,6,8,10,12,14,16\}, D=\{5,10,15,20\} ;$ find

$A-C$

If $aN = \{ ax:x \in N\} ,$ then the set $3N \cap 7N$ is .....$N$

Let $P=\{\theta: \sin \theta-\cos \theta=\sqrt{2} \cos \theta\}$ and $Q=\{\theta: \sin \theta+\cos \theta=\sqrt{2} \sin \theta\}$ be two sets. Then

If $A, B$ and $C$ are any three sets, then $A -(B  \cup C)$  is equal to