If the width is w and the length is l, we have l/2=w and 2w+2l=54 (given the formula for the perimeter of a rectangle). Plugging l/2=w into the second equation, we have 2(l/2)+2l=54=l+2l=3l. Dividing both sides by 3, we get l=18. Since w=l/2, w=18/2=9