You're going to divide the mass of chlorine within the compound by the mass of the compound, and then multiply the result by 100 to get the answer
74.62 g of magnesium oxide is formed from 45.00 g magnesium so 74.62-45.00= 29.62 g of oxygen is consumed or in other words a new compound is formed in the burning of magnesium in oxygen with a heavier mass than the pure magnesium.
It makes the bronze stronger and harder than either of the other two medals