Unless otherwise provided in the articles of incorporation, a majority of the shares entitled to vote, represented in person or by proxy, shall constitute a quorum at a meeting of shareholders, but in no event shall a quorum consist of less than one-third of the shares entitled to vote at the meeting. A quorum, once attained at a meeting, shall be deemed to continue until adjournment notwithstanding the voluntary withdrawal of enough shares to leave less than a quorum. If a quorum is present, the affirmative vote of the majority of the shares represented at the meeting and entitled to vote on the subject matter shall be the act of the shareholders, unless the vote of a greater number or voting by classes is required by the Business Corporation Act or the articles of incorporation.