The perimeter of a rectangle is the sum of its sides, which are two widths and two lengths:
[tex]p=w+w+l+l[/tex]
Since the length is twice the width, we have [tex]l=2w[/tex], and the formula for the perimeter becomes
[tex]p=w+w+2w+2w=6w[/tex]
So, we have
[tex]6w=120 \iff w=\dfrac{120}{6}=20[/tex]
