Define the centre of mass.

The centre of mass of a system of particles is a unique point where the entire mass of the system can be considered to be concentrated for the purpose of describing its translational motion.
Mathematically,for a system of $n$ particles with masses $m_1, m_2, ..., m_n$ located at position vectors $\vec{r}_1, \vec{r}_2, ..., \vec{r}_n$,the position vector of the centre of mass $\vec{R}$ is given by:
$\vec{R} = \frac{\sum_{i=1}^{n} m_i \vec{r}_i}{\sum_{i=1}^{n} m_i} = \frac{1}{M} \sum_{i=1}^{n} m_i \vec{r}_i$
where $M = \sum m_i$ is the total mass of the system.