Mr. Salam has an open terrace adjacent to his apartment where his children play football. To protect the children from falling, Mr. Salam wants to put up railing around three sides of his rectangular terrace and leave a side of 20 feet untended as that side is covered by his apartment and needs no railing. If the terrace has an area of 680 square feet, how many feet. how many feet of railing does he need?