Since the major axis is 80 yards long, the distance from the center to a vertex on the major axis, which is the "a" in the equation, would be 40 yards. With similar logic we can find that the distance from the center to a vertex on the minor axis, "b" in the equation, would be 36 yards.
With the center, a and b we are just about ready to write the equation. The standard forms for equations of ellipses are:
(x-h)^2 / a^2 + (y-k)^2 / b^2 = 1 for ellipses with horizontal major axes and
x-h)^2 / b^2 + (y-k)^2 / a^2 = 1 for ellipses with vertical major axes
Since the major axis is the x-axis, which is horizontal, we will use the first form. Using the values we found for a and b and the x-coordinate of the center as "h" and the y-coordinate of the center as "k" we get:
(x-0)^2 / (40)^2 + (y-0)^2 / (36)^2 = 1
which simplifies to:
x^2 / 1600 + y^2 / 1296 = 1