Answer: 766.41m
Explanation:
Hi, since the situation forms 2 right triangles (see attachment) we have to apply the Pythagorean Theorem for one triangle.
c^2 = a^2 + b^2
Where c is the hypotenuse of the triangle (in this case the diagonal distance between corners of the field, which is the distance between the posts if they are placed as far apart as possible ) and a and b are the other sides.
Replacing with the values given:
c^2 = 616^2 + 456^2
c^2 = 379,456+207,936
c^2 =587,392
c = √587,392
c = 766.41m
So, she is within the area serviced by the tower (20 miles)
Feel free to ask for more if needed or if you did not understand something.