Question

What is the l and w of a rectangular field of 40 ft perimeter?

Answer 1

The perimeter doesn't tell you the dimensions.

With a 40-ft perimeter, there an infinite number of possibilities
for the length and with.  The only thing we know for sure is that
the sum of (length + width) is 20-ft.

The field could be . . .

1'  by  40'
2'  by  20'
3'  by  13-1/3 '
4'  by  10'
5'  by  8'
6'  by  6-2/3 '