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

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

$A=\{a, e, i, o, u\} B=\{a, b, c\}$

Consider the following relations :

$(1) \,\,\,A - B = A - (A \cap B)$

$(2) \,\,\,A = (A \cap B) \cup (A - B)$

$(3) \,\,\,A - (B \cup C) = (A - B) \cup (A - C)$

which of these is/are correct

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 $\mathrm{R}$ is the set of real numbers and $\mathrm{Q}$ is the set of rational numbers, then what is $\mathrm{R - Q} ?$

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

$A=\{a, e, i, o, u\} B=\{a, b, c\}$