A majority of the actual membership of the council plus the mayor, excluding vacant seats, shall constitute a quorum. A member who has withdrawn from a meeting without being excused by majority vote of the remaining members present shall be counted as present for purposes of determining whether or not a quorum is present. (1917, c. 136, subch. 13, s. 1; C.S., s. 2821; 1971, c. 698, s. 1; 1975, c. 664, s. 5; 1979, 2nd Sess., c. 1247, s. 6.)