The centre of mass is a point:

Three identical spheres of radius $r$ are placed on a horizontal surface such that they touch each other. These spheres are arranged such that their centers form an equilateral triangle. The center of mass of this system will be at ........

Four particles of masses $1\,kg, 2 \,kg, 3 \,kg$ and $4\, kg$ are placed at the four vertices $A, B, C$ and $D$ of a square of side $1\, m$ as shown in the figure. The coordinates of the centre of mass of the particles are:

Three identical spheres,each of mass $1 \ kg$,are arranged as shown in the figure. If their centers of mass are at $P$,$Q$,and $R$ respectively,what is the distance of the center of mass of this system from point $P$?

$A$ system consists of $3$ particles each of mass $m$ located at $(1, 1), (2, 2),$ and $(3, 3)$. The coordinates of the centre of mass are

To find the centre of mass of a rigid body,why is it not possible to know $\sum m_i \vec{r}_i$ for all the particles?