[planke]

Can someone help me with this problem?

Consider a 5.319g mixture of FeO and Fe3O4. You react this mixture with an excess of oxygen to form 5.731g Fe2O3. Calculate the percent by mass of FeO in the original mixture.

[Borek]

You have to write two simultaneous equations - one for mass of the sample before oxidation, one for mass of the sample after oxidation. Your unknonws - masses (or numbers of moles) of both oxides before oxidation. Your source of necessary stoichiometric information - two balanced reaction equations.