In a solution of 225 mM sodium phosphate buffer (so the concentration represents all forms of phosphate no matter the ionization state) at pH 6.9, what is the ratio of HPO4^-2 to H2PO4^-? The pKa's of phosphoric acid are 2.15, 6.86, and 12.32. The answer is unitless, but report your answer to the nearest 0.01.