If a quorum is not specified in the Articles of Incorporation or Bylaws, then a quorum is the a majority of the numbers of directors specified in the bylaws or Articles - or more than half. If the corporation chooses, it can specify a lower number of directors for a quorum in its Bylaws or Articles of Incorporation, although the lowest allowable quorum is at least 1/3 of those specified in the Articles.
While the rule is similar, it is important to know what kind of corporation are you speaking of - profit or nonprofit. A profit corporation is governed by the Washington Business Corporation Act - RCW Chapter 23. RCW Chapter 24, the one you are quoting, specifically deals with nonprofit corporations and assocaitions.
If you are speaking of a for profit corporation, then a quorum of Directors is defined in:
Quorum and voting.
(1) Unless the articles of incorporation or bylaws require a greater or lesser number, a quorum of a board of directors consists of a majority of the number of directors specified in or fixed in accordance with the articles of incorporation or bylaws.
(2) Notwithstanding subsection (1) of this section, a quorum of a board of directors may in no event be less than one-third of the number of directors specified in or fixed in accordance with the articles of incorporation or bylaws.
Note that this is NOT the same quorum requirement for shareholders, which is specified in RCW 23B.07.250.