After chemical reaction with oxygen only, the ignition of MnO2 in air converts it quantitatively to Mn3O4. A 100g sample of pyrolusite contains 80% (by mass) of MnO2, 15% SiO2 and the remaining parts are only water. This sample is heated in air to final constant mass. Calculate the percentage mass of Mn in the ignited sample. SiO2 is very heat-resistant even at the maximum ignition temperature.