Mark M. answered 11/17/20
Retired math prof. Very extensive Precalculus tutoring experience.
Let x = length of each subdivision and y = length of each of the other sides
2y + 5x = 1200 So, y = (1200 - 5x) / 2
Area = A = xy = (1/2)(x)(1200 - 5x) = (1/2)(-5x2 + 1200x)
A' = (1/2)(-10x + 1200) = 0 when x = 120
Maximum area when x = 120
Maximum area = (1/2)(120)(600) = 36000 m2