This question belongs to greatest common factor.
Given, there are two types of trees.
25 elm trees and 20 oak trees.
Let's find the factors of 25 and 20
25 = 5 * 5
20 = 5* 4
The common factor between both the numbers is 5.
Hence, there should be 5 areas so that the trees can be planted equally.
Answer = 5 areas.