length = 5x 
width = x 
area = 125 
A = L*W 
125 = (5x)(x) 
125 = 5x^2 
125/5 = x^2 
25 = x^2 
sqrt{25} = sqrt{x^2} 
5 = x 
The width is 5 feet. 
The length is 5 times 5 feet or 25 feet. 
To find the perimeter, we use P = 2L + 2W. 
P = 2(25) + 2(5) 
P = 50 + 10 
P = 60 feet. 
Done!