8. (10 points) On the way from Xiaoming's house to the swimming pool, there are 200 trees. On the round trip, Xiaoming used red ribbons to mark some trees. On the way to the swimming pool, he marked the 1st tree, the 6th tree, the 11th tree, ... each time marking a tree after skipping 4 trees; on the way back, he marked the 1st tree, the 9th tree, the 17th tree, ... each time marking a tree after skipping 7 trees. When he returned home, the number of trees that were not marked is $\qquad$ trees.
【Analysis】According to the problem, when going to the swimming pool, every 5 trees are marked with one, a total of $200 \div 5=40$ trees; on the way back, every 8 trees are marked with one, a total of $200 \div 8=25$ trees; the number of trees marked repeatedly is $200 \div (5 \times 8)=5$ trees. Adding 40 and 25, and then subtracting 5, we can find out how many trees were marked in total. Finally, subtracting the number of marked trees from 200, we can find out how many trees were not marked.

【Solution】Solution: When going to the swimming pool, every 5 trees are marked with one,
a total of $200 \div 5=40$ (trees);
on the way back, every 8 trees are marked with one,
a total of $200 \div 8=25$ (trees);
the number of trees marked repeatedly is $200 \div (5 \times 8)=5$ (trees),
$$
\begin{array}{l}
200-(40+25-5) \\
=200-60 \\
=140 \text { (trees) }
\end{array}
$$

Answer: The number of trees that were not marked is 140.
The answer is: 140.