The perimeter of a triangle has the formula 2l + 2w. Since the length is twice as long as the width, l = 2w. We use this equations to two possible perimeters, 90 and 120 feet.
P = 90 feet
90 = 2(2w) + 2w
90 = 6w
w = 15 feet
P = 120 feet
120 = 2(2w) + 2w
120 = 6w
w = 20
Thus, the range of w is from 15 to 20 feet. The answer is 15 ≤ w ≤ 20