Question

If $n\left(A\right)=15,n(A\cup B)=29,n(A\cap B)=7$ then find $n\left(B\right)$.

Answer 1

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

$\therefore n\left(B\right)=29+7-15$

$=21$.