Question: Let $A$ and $B$ be two sets that $n(A)=16, n(B)=14, n(A \cup B)=25$. Then, $n(A \cap B)$ is equal to
(a) 30
(b) 50
(c) 5
(d) none of these
Solution:
We know:
$n(A \cup B)=n(A)+n(B)-n(A \cap B)$
Now,
$n(A \cap B)=n(A)+n(B)-n(A \cup B)$
$=16+14-25$
$=5$
