\displaystyle \displaystyle \mathbf{r_g} = \frac{200\textrm{g}(0,2)\textrm{m} + 100\textrm{g}(0,-2)\textrm{m}}{200\textrm{g}+100\textrm{g}} = \frac{(0,200)}{300}\textrm{m} = \frac{(0,2)}{3}\textrm{m}
