Question

If $n\left(A\right)=15,n(A-B)=10$ then find $n(A\cap B)$

Answer 1

We have $n(A-B)+n(A\cap B)=n\left(A\right)$

$\Rightarrow 10+n(A\cap B)=15$

$\Rightarrow n(A\cap B)=15-10=5$

$\therefore n(A\cap B)=5$