Explanation

(c) There is a point in the system, where if whole mass of the system is supposed to be concentrated, the nature of the motion executed by the system remains unaltered when the force acting on the system are applied directly at this point.
The position of centre of mass of system for $n$ particles is expressed as
$R_{CM} = \frac{\sum_{}^{}{m_{i}r_{i}}}{\sum_{}^{}m_{i}}$
or $\sum_{}^{}{m_{i}r_{i}} = constant$
Hence, for a system having particles, we have
$m_{1}r_{1} = m_{2}r_{2}$
$\Rightarrow$ $\frac{r_{1}}{r_{2}} = \frac{m_{2}}{m_{1}}$
$ie$, the centre of mass of a system of two particle divides the distance between them in inverse ratio of masses of particles.