Total surface area of a triangular prism = width x height +(sum of sides + width) x length
475 = (width x 10) + (5 + 5 + width) x 15
475 = (10 x width) + (10 + width) x 15
475 = (10 x width) + 150 + (15 x width)
475 - 150 = 25 x width
325 = 25 x width
width = 325/25 = 13 feet
