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
$D-B$
Let $A=\{a, b\}, B=\{a, b, c\} .$ Is $A \subset B \,?$ What is $A \cup B \,?$
If $A$ and $B$ are two sets, then $A \cup B = A \cap B$ iff
Show that $A \cap B=A \cap C$ need not imply $B = C$
If $aN = \{ ax:x \in N\} ,$ then the set $3N \cap 7N$ is .....$N$