Calculate the mass of the reactant $\mathrm{A}$ in a reaction where the reactants $\mathrm{A}$ and $\mathrm{B}$ combine together to form a product $\mathrm{AB}$. The mass of $\mathrm{B}$ is $10 \mathrm{g}$ and the mass of $\mathrm{AB}$ is $24 \mathrm{g}$.