If  $n\left(A\cup B\cup C\right)=50,n\left(A\right)=15,n\left(B\right)=21,n\left(C\right)=18$ , $n\left(A\cap B\right)=5,n\left(B\cap C\right)=6,n\left(C\cap A\right)=7$ , then $n\left(A\cap B\cap C\right)=$

$n\left(A\cap B\right)=5,n\left(B\cap C\right)=6,n\left(C\cap A\right)=7$

$n\left(A\cup B\cup C\right)=n\left(A\right)+n\left(B\right)+n\left(C\right)-n\left(A\cap B\right)-n\left(B\cap C\right)-n\left(C\cap A\right)$

                                                                                                 $+n\left(A\cap B\cap C\right)$                                                                                                                                                             

$50=15+21+18-\left(5+6+7\right)+n\left(A\cap B\cap C\right)$

$50=54-18+n\left(A\cap B\cap C\right)$

$n\left(A\cap B\cap C\right)=50-36$

$n\left(A\cap B\cap C\right)=14$