Complement of a Set

$n(A\cup B\cup C)+n(A\cup B\cup C{)}^{\mathrm{\text{'}}}=50$

$n(\cup )=50$

$n(A\cup B\cup C{)}^{\mathrm{\text{'}}}=n(\cup )-n(A\cup B\cup C)\Rightarrow 50-29\Rightarrow 21$

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

$\Rightarrow \mathrm{}10+15+20-5-6-10-5\Rightarrow 29$

$n(A\cup B\cup C{)}^{\mathrm{\text{'}}}=21$