Answer:
Width = 20 ft
Step-by-step explanation:
Let us call w the width and l the length of the rectangle, then we know that

in other words, the perimeter is 120 ft.
Furthermore, we also know that the length of the rectangle is twice the width

substituting this value into the first equation gives



Finally, dividing both sides by 6 gives


Hence, the width of the rectangle is 20 ft.