Question:
If $A$ and $B$ are two sets such that $n(A)=115, n(B)=326, n(A-B)=47$, then write $n(A \cup B)$.
Solution:
$n(A)=115, n(B)=326$ and $n(A-B)=47$
Now,
$n(A)-n(A \cap B)=n(A-B)$
$\Rightarrow 115-n(A \cap B)=47$
$\Rightarrow n(A \cap B)=68$
Thus, we get:
$n(A \cup B)=n(A)+n(B)-n(A \cap B)$
$=115+326-68$
$=373$
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