what are the length and width?

the length of a rectangle is twice as great as it's width. when both width and length are increased by 5 inches, the new rectangle has a perimeter of 80 inches. write an equation to find the length and width of the original rectangle.

the length of a rectangle is twice as great as it's width:
 
l = 2w
 
when both width and length are increased by 5 inches, the new rectangle has a perimeter of 80 inches
 
2(l+5)+ 2(w+5) = 80
 
substitute 2w for l:
 
2(2w+5) + 2(w+5) = 80
 
4w + 10 + 2w + 10 = 80
 
6w + 20 = 80
 
6w = 60
 
w = 10 inches
 
l = 2w = 2*10 = 20 inches