If $A=\left\{1, 2, 3, 4\right\}, B=\left\{3, 4, 5, 6\right\}$ and $C=\left\{1, 3, 7, 9\right\},$find:

$A\cup B$

If $A=\left\{1, 2, 3, 4\right\}, B=\left\{3, 4, 5, 6\right\}$ and $C=\left\{1, 3, 7, 9\right\},$find:

$A\cup C$

If $A=\left\{1, 2, 3, 4\right\}, B=\left\{3, 4, 5, 6\right\}$ and $C=\left\{1, 3, 7, 9\right\},$find:

$A\cap B$

If $A=\left\{1, 2, 3, 4\right\}, B=\left\{3, 4, 5, 6\right\}$ and $C=\left\{1, 3, 7, 9\right\},$find:

$\left(A\cup B\right)\cap \left(A\cup C\right)$

Given $\xi =\{x | x\in N$ and $x\le 9\}, A= \left\{2, 3, 4, 6\right\}, B=\left\{1, 3, 4, 9\right\}$, find

$A\text{'}$

Given $\xi =\{x | x\in N$ and $x\le 9\}, A= \left\{2, 3, 4, 6\right\}, B=\left\{1, 3, 4, 9\right\}$, find:

$\left(A\cup B\right)\text{'}$

Given $\xi =\{x | x\in N$ and $x\le 9\}, A= \left\{2, 3, 4, 6\right\}, B=\left\{1, 3, 4, 9\right\}$, find: 

$\left(A\cap B\right)\text{'}$

Given $\xi =\{x | x\in N$ and $x\le 9\}, A= \left\{2, 3, 4, 6\right\}, B=\left\{1, 3, 4, 9\right\}$, find:

$A\text{'} \cup B\text{'}$

If $n\left(A\right)=20, n\left(B\right)=10$ and $n\left(A\cap B\right)=4$, find $n\left(A\cup B\right)$.