The formula for the centre of mass of a two-particle system is
1
\({r_{cm}} = \frac{{{r_1} + {r_2}}}{{{m_1} + {m_2}}}\)
2
\({r_{cm}} = \frac{{{m_1}{r_1} + {m_2}{r_2}}}{{{m_1} + {m_2}}}\)
3
\({r_{cm}} = \frac{{2\left( {{r_1} + {r_2}} \right)}}{{{m_1} + {m_2}}}\)
4
\({r_{cm}} = \frac{{{m_1}{r_1} + {m_2}{r_2}}}{2}\)