$${n = \frac{(2000)(200\, \mathrm{kg/m^3})(6,02 \times 10^{23}\, \mathrm{mol^{-1}})}{2,0 \times 10^{-3}\, \mathrm{kg/mol} + 3,0 \times 10^{-3}\, \mathrm{kg/mol}} \\ \underline{\underline{4,8 \times 10^{31}\, \mathrm{m^{-3}}}}}$$