What is a system of particles?

Choose the correct statement about the centre of mass $(CM)$ of a system of two particles.

$A$ uniform thin rod $AB$ of length $L$ has linear mass density $\mu(x) = a + \frac{bx}{L}$,where $x$ is measured from $A$. If the $CM$ of the rod lies at a distance of $\frac{7}{12}L$ from $A$,then $a$ and $b$ are related as

On a horizontal frictionless frozen lake,a girl of mass $36 \,kg$ and a box of mass $9 \,kg$ are connected to each other by means of a rope. Initially,they are $20 \,m$ apart. The girl exerts a horizontal force on the box,pulling it towards her. How far has the girl travelled when she meets the box?

$Assertion$ : The position of the centre of mass of a body depends upon the shape and size of the body.
$Reason$ : The centre of mass of a body always lies at the centre of the body.

For a system consisting of two objects,when can they be treated as point particles?