How can a general body be treated in mechanics?

Define the centre of mass.

If two bodies of masses $2 \,kg$ and $3 \,kg$ are moving at right angles with velocities $20 \,m \,s^{-1}$ and $10 \,m \,s^{-1}$ respectively, then the velocity of the centre of mass of the system of the two bodies is

Three identical spheres,each of mass $1 \ kg$,are kept as shown in the figure,touching each other,with their centres on a straight line. If their centres are marked $P, Q, R$ respectively,the distance of the centre of mass of the system from $P$ is

Two point masses $M$ each are placed at $(L, 0)$ and $(-L, 0)$. $A$ third point mass $M$ is uniformly rotating on the circle $x^2 + y^2 = L^2$. The equation of the path traced by the $COM$ of the $3$ point masses is:

$A$ solid cylinder of mass $2 \ kg$,length $40 \ cm$ and radius $10 \ cm$ is placed in contact with a solid sphere of mass $0.5 \ kg$ and radius $10 \ cm$ such that the centres of the two bodies lie along the geometrical axis of the cylinder. The distance of the centre of mass of the system of two bodies from the centre of the sphere is (in $cm$)