Question 969292
Let a be the side of a square plot
perimeter = 320 feet
4a = 320
a=320/4 = 80
a=80
by Pythagorean theorem  
a^2 + a^2 =(diagonal)^2
80^2 + 80^2 = (diagonal)^2
6400 + 6400 =(diagonal)^2
12800 = (diagonal)^2
diagonal = sq rt(12800) = 11.314 
diagonal = 11.314