Answer:
502
Explanation:
It depends on the arrangement.
If a tree is planted in the middle of each 20 m space, then you get one answer. If a tree is planted at the ends of each 20 m space, then you get a different answer.
Here, we will assume that a tree is planted at the ends of the road and every 20 m in between. Then the number planted on one side of the road is ...
1 + (5000 m)/(20 m) = 251
And the number of trees planted on both sides of the road is ...
2 × 251 = 502 . . . trees planted altogether
_____
Additional comment
If _ represents a 10 m space, the arrangements we're talking about above are ...
_T_ . . . tree in the middle of the space
T_ _ T . . . trees at the end of the space
Knowing that a tree requires room to spread out, we might choose the former arrangement. That would mean a total of 500 trees, rather than 502.