The acts of the board shall be expressed by motion, resolution or ordinance. No ordinance shall be passed until a public hearing has been held on it, which hearing shall be advertised in a newspaper of general circulation or posted in at least three public places at least 15 days prior to the hearing. No ordinance shall be adopted by the board on the day of its introduction, and no ordinance, resolution or motion shall have any validity or effect unless passed by the affirmative votes of a majority of the directors.