Section 55. The board shall hold at least four regular meetings each year and may hold special meetings as required. Time, place and notice of all meetings shall be as required by rules or by-laws made by the board. A quorum shall consist of three members. In case of vacancy in the office or absence of the chairman the senior member shall perform the duties of the chairman. A written record which shall be open to public inspection shall be kept of all meetings and of the business transacted thereat.