Amounts of tin may be alloyed with copper to produce bronzes of varying composition and properties. 7.7304 g of a particular bronze, compounded of only Cu and Sn, can be quantitatively converted into a 9.6999-g mixture of the oxides CuO and SnO₂. (Use the atomic masses: Cu 63.546, Sn 118.71, O 15.9994.) What was the percentage (by mass) of tin in the alloy?