Question:
If $A$ and $B$ are two sets such that $n(A)=20, n(B)=25$ and $n(A \cup B)=40$, then write $n(A \cap B)$.
Solution:
We have:
$n(A)=20, n(B)=25$ and $n(A \cup B)=40$
We know:
$n(A \cup B)=n(A)+n(B)-n(A \cap B)$
$\Rightarrow n(A \cap B)=n(A)+n(B)-n(A \cup B)$
$=20+25-40$
$=5$