Answer: We get that 100-l represents the breadth of the pen.
Explanation:
Since we have given that
Perimeter of rectangle = 200 feet
Let the length of the pen for her pigs be 'l'.
So, According to question,
Perimeter of rectangle = 2(l+b)

And we know the formula for "Area of rectangle":

So, we get that 100-l represents the breadth of the pen.