Question 548225
If we were to draw a picture we would see that we have a right-angled triangle. 

This means we can use Pythagorean Theorem to determine the length of the diagonal (which in our case is the hypotenuse).  

{{{a^2+b^2=c^2}}}
{{{120^2+80^2=c^2}}}
{{{14400+6400=c^2}}}
{{{20800=c^2}}}

To eliminate the square, we must take the square root of both sides leaving:
{{{c=sqrt(20800)}}}
This is around 144.22.  Therefore the diagonal is around 144.22 yards in length.