15. (6 points) 4 workers can produce 108 parts in 3 hours. Now, to produce 504 parts in 8 hours, the number of additional workers needed is $\qquad$.

【Solution】Solution: $504 \div 8 \div(108 \div 3 \div 4)-4$,
$$
\begin{array}{l}
=504 \div 8 \div 9-4, \\
=63 \div 9-4, \\
=7-4, \\
=3 \text { (people), }
\end{array}
$$

Answer: Need to add 3 people, so the answer should be: 3 .