$0.920\mathrm{g}$ of a mixture of $\mathrm{CaCO}{}_3$ (molar mass 100) and $\mathrm{MgCO}{}_3$ (molar mass 84) is heated with $50.0\mathrm{mL}$ of $0.500\mathrm{M}$ $\mathrm{HCl}$. The excess acid needs $5.0\mathrm{mL}$ of $1.00\mathrm{M}$ $\mathrm{NaOH}$. The mass percent of $\mathrm{CaCO}{}_3$ in the mixture is

HCl consumed = 0.0250 - 0.0050 = 0.0200 mol; each carbonate uses 2 HCl, so total carbonate = 0.0100 mol. Let x = mol CaCO3: 100x + 84(0.0100 - x) = 0.920, so 16x = 0.080 and x = 0.0050. Mass CaCO3 = 0.500 g; percent = 54.35%.