Find the intersection of each pair of sets :

$X=\{1,3,5\} Y=\{1,2,3\}$

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

$B-D$

Find the union of each of the following pairs of sets :

$A = \{ x:x$ is a natural number and multiple of $3\} $

$B = \{ x:x$ is a natural number less than $6\} $

If $n(A) = 3$ and $n(B) = 6$ and $A \subseteq B$. Then the number of elements in $A \cap B$ is equal to

Let $A, B$ and $C$ be sets such that $\phi  \ne A \cap B \subseteq C$. Then which of the following statements is not true ?