Answer
$18$
Work Step by Step
$A\cap \overline{C}$ is the set of elements that are in $A$ but not in $C$.
Add all the numbers inside the circle that represents set $A$ but is outside the circle thar represents $C$.
Using the diagram, the number of elements in $A$ but not $C$ or $n(A\cap \overline{C})=15+3=18$
