Question:
If $A$ and $B$ are two sets such that $n(A)=70, n(B)=60, n(A \cup B)=110$, then $n(A \cap B)$ is equal to
(a) 240
(b) 50
(c) 40
(d) 20
Solution:
(d) 20
We have:
$n(\mathrm{~A} \cap \mathrm{B})=n(\mathrm{~A})+n(\mathrm{~B})-n(\mathrm{~A} \cup \mathrm{B})$
$=70+60-110$
$=20$