\displaystyle \displaystyle \mathbf{r}_g = \frac{4\textrm{kg}(0,5;\,0,5)\textrm{m}+4\textrm{kg}(0,5;\,-0,5)\textrm{m}}{4\textrm{kg}+4\textrm{kg}} = \frac{4(1,0;\,0)}{8}\textrm{m} =(0,5;\,0)\,\textrm{m}
