The centre of mass of a body

Three identical spheres of mass $1 \ kg$ each are arranged as shown in the figure. The centers of the spheres,which are touching each other,lie on a straight line. If their centers are denoted by $P, Q,$ and $R$,what is the distance of the center of mass of the system from $P$?

Infinite numbers of blocks are placed on a table edge as shown in the diagram. What is the minimum value of $L$ so that the blocks are just going to topple over the table?

Three rods of equal mass are arranged as shown in the figure. Find the coordinates of the center of mass of the system.

If the linear mass density of a rod of length $L$ varies as $\lambda = kx^3$,determine the position of its centre of mass (where $x$ is the distance from one of its ends and $k$ is a constant):

Five masses are placed in a plane as shown in the figure. The coordinates of the centre of mass are nearest to: