## Thinking Mathematically (6th Edition)

$n(A\cap B)[n(A\cup B)-n(A')]=2$
$n(A\cap B)$ is the number of elements of $A$ that are also elements of $B$. We can see that $n(A\cap B)=2$ $n(A\cup B)$ is the number of distinct elements which are in $A$, in $B$, or in both $A$ and $B$. We can see that $n(A\cup B)=7$ n(A') is the number of elements in the diagram that are not in $A$. We can see that $n(A')=6$ Therefore: $n(A\cap B)[n(A\cup B)-n(A')]=2*(7-6)=2$