Answer:
80 feet by 20 feet
Explanation:
Area of the rectangular enclosure = 1600ft²
Area, lw=1600
w=1600/l
Perimeter, 2w+l=120 feet
Substituting w=1600/l into the perimeter
2(1600/l)+l=120
3200/l+l=120

When l=40, w=1600/40=40 feet
When l=80, w=1600/80=20 feet
Since the enclosure is rectangular, the acceptable dimensions are:
Length =80 feet
Width =20 feet