【Question 7】
On Tree Planting Day, the students of Class 4(1) went to the park to plant trees. Along a 120-meter long road on both sides, they dug a hole every 3 meters. Later, due to the spacing being too small, they changed it to digging a hole every 5 meters. In this way, at most $\qquad$ holes can be retained.
【Analysis and Solution】
If the distance from the starting point is a multiple of both 3 and 5, then this pit can be retained; $[3,5]=15$ meters; $\square$
$120 \div 15=8$ segments;
According to the "linear two-sided tree planting problem", number of trees = number of segments + 1; and pits are dug on both sides of the road;

The maximum number of pits that can be retained is $(8+1) \times 2=18$.