the width of the deck woul be 5 feet because, the deck is a rectangle, meaning it has 4 sides. If you know two of thoses sides are 10 feet in lenghth, that adds up to 20 ft in perimeter. You subtract 20 from the perimeter of 30 ft, and get 10. You divide the ten by the two remaining sides of the deck and get the answer of 5 ft
