\displaystyle \displaystyle \mathbf{r_g} = \frac{200\textrm{g}(1,1)\textrm{m} + 100\textrm{g}(2,-1)\textrm{m}}{200\textrm{g}+100\textrm{g}} = \frac{(400,100)}{300}\textrm{m} = \frac{(4,1)}{3}\textrm{m}
