Hi there! The answer is 400 square yards.
When the total fencing around a square field is 80 yards, that means that the fencing on one side of the square field is 80 / 4 = 20 yards (Since a square field has 4 sides which are equally as long as every other side).
When the side of the field is 20 yards, the area (which is width * length) equals 20 yards * 20 yards = 400 square yards.
