15. (5 points) As shown in the figure, the side length of square $A B C D$ is 8 cm, and the side length of square $D E F G$ is 5 cm. Then the area of triangle $A C F$ is $\qquad$ square centimeters.
【Solution】Solution: $8+5=13$ (cm)
$$
\begin{array}{l}
13 \times 13-13 \times 5 \div 2 \times 2-8 \times 8 \div 2 \\
=169-65-32 \\
=72 \text { (square cm) }
\end{array}
$$

Answer: The area of triangle $ACF$ is 72 square cm. Therefore, the answer is: 72.