Each representative in attendance at a meeting of the Board of Supervisors at which a quorum is present shall cast one vote for every 2,500 population, or part thereof, in the municipality which he or she represents. If a representative is unable to attend, the municipality's duly designated alternate shall be entitled to vote. A member of the Board of Supervisors may not split his or her votes. In order for any motion to carry, it must be voted on affirmatively by a minimum of the representatives from one-third of the member municipalities and a majority of the votes cast at a meeting of the Board of Supervisors, except as otherwise provided in this chapter.