If a substance '$A$' dissolves in solution of a mixture of '$B$' and '$C$' with their respective number of moles as $\mathrm{n}_{\mathrm{A}}, \mathrm{n}_{\mathrm{B}}$ and $\mathrm{n}_{\mathrm{C}_3}$. Mole fraction of $\mathrm{C}$ in the solution is