Question 823767
Knowing that the biggest area occurs with x and y of a rectangle being equal, the big pen would be square, x=y, or side of just x.  ASSUMING the divider is parallel to one of the sides, then fencing length is 5x.


5x=100
x=20 feet.
The whole-thing area is 400 square feet.


The rest of your question does not seem fitting to the problem.