Define the position vector of the centre of mass.

(N/A) The position vector of the centre of mass $\vec{R}$ for a system of $n$ particles with masses $m_1, m_2, ..., m_n$ located at position vectors $\vec{r}_1, \vec{r}_2, ..., \vec{r}_n$ is defined as the weighted average of the position vectors of all the particles in the system.
Mathematically,it is given by:
$\vec{R} = \frac{\sum_{i=1}^{n} m_i \vec{r}_i}{\sum_{i=1}^{n} m_i} = \frac{m_1 \vec{r}_1 + m_2 \vec{r}_2 + ... + m_n \vec{r}_n}{M}$
where $M = \sum_{i=1}^{n} m_i$ is the total mass of the system.