The perimeter of a rectangle is 18 feet, and the area of the rectangle is 20 square feet. what is the length of the rectangle?

If the width = w and length = l we have

lw = 20       (area)

and 2l + 2w = 18    (perimeter)

which is same as l + w = 9  

from first equation w = 20/ l   so substituting in l + w = 9:-

20/ l  + l  = 9 

20 + l^2 = 9l
l^2 - 9l + 20  = 

(l - 5)(l - 4) = 0

so l is 5 or 4

But it must be 5 because length must be greater than width

So answer is length = 5 feet