Using your measured mass of Mg and your measured mass of MgO, calculate the percentage composition of MgO based on your data. (I feel like my data is wrong, could someone please explain this to me.)

In my lab, I measured Mg as .1 g and MgO was also .1 g, I thought this was right due to the law of conservation of mass which states that the mass will stay the same before and after a chemical reaction, but I cannot figure out what to do for the percent composition. Please help