The equation of that ellipse is

2a = Major axis = 80, so a = semi-minor axis = 40
2b = Minor axis = 40, so a = semi-minor axis = 20
 
Equation is of the form:



or






To find the width 10 feet from the vertex, we want to
know the length of this green line:


 
We want to know the value of y when x = 30, so we substitute
30 for x in









Multiply through by 1600













So the endpoints of that green line are

 and 

and its length is  or about 26.46 feet.

Edwin