Answer is: The mass of the product of reaction (iron(III) oxide equals the mass of steel wool and the oxygen.
Balanced chemical reaction of forming rust: 4Fe + 3O₂ → 2Fe₂O₃.
Conservation of mass (mass is never lost or gained in chemical reactions), during chemical reaction no particles are created or destroyed, the atoms are rearranged from the reactants to the products.
Conservation of Matter states that the reactants have to equal the products.