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 '