I'm trying to follow what the question is asking; it doesn't seem perfectly clear. I think you should take all the oxides and count those towards your abundances. So, for example you have 19000 Oxygen atoms per Silicon atom. And oxygen appears in CaO, Al2O3, Na2O, MgO,Al2O3, and SiO2. So maybe you need to divide the available abundance of oxygen up into these oxides.
But this question is somewhat outside of my experience.