Stoichiometry of a Mixture Decomposition

A 0.500 g sample containing a mixture of MgCO₃ and CaCO₃ is heated to complete decomposition according to the reaction: $\mathrm{MCO}_3(s) \rightarrow \mathrm{MO}(s) + \mathrm{CO}_2(g)$. The decomposition produces 0.224 g of CO₂. What is the mass percentage of MgCO₃ in the original mixture?