If the width of a rectangle is 4 feet less than the length and the area is 320 feet squared what is the width of the rectangle?

20 feet long by 16 feet wide
Let the width be x-4 and the length be x:

(x-4)*x = 320

x2-4x-320 = 0

Solving the above quadratic equation works out as:

x = -16 or x = 20 so it must be the latter because dimensions can't be negative.

Answer: width = 16 feet