8. (3 points) There are 10 pencil cases, among which 5 contain pencils, 4 contain pens, and 2 contain both pencils and pens. The number of empty pencil cases is $\qquad$.
【Answer】Solution: $10-(5+4-2)$,
$$
\begin{array}{l}
=10-7, \\
=3 \text { (boxes); }
\end{array}
$$

Answer: There are 3 empty pencil boxes;
Therefore, the answer is: 3.