Question

If $n\left(\xi \right)$ $=30,n\left(A\right)=22,n\left(B\right)=15$ and $n(A$ $\cup$ $B)=25$, then find : $n(B$${}^{\prime }$$)$.

Answer 1

$n\left(\xi \right)=30$

$n\left(A\right)=22$

$n\left(B\right)=15$

$n(A\cup B)=25$

$n\left(B^{\prime }\right)=n\left(\xi \right)-n\left(B\right)$

$=30-15$

$=15$

$n\left(B^{\prime }\right)=15$