Ans.

A particle of mass $m$ collides head-on with another particle of mass $2m$.

The velocity of the particle of mass $m$ is $V$,
and the velocity of the particle of mass $2m$ is zero.

The collision is perfectly inelastic. after collision the two bodies together behave like one body, so the velocity after collision of the two bodies will be same. Let $V_1$ be the velocity of the composite particle after collision.

According to the conservation of linear momentum,

$mV+2m\cdot{0}=(m+2m)V_1$

or, $mV=3mV_1$

or, $V_1=\frac{1}{3}V$

So the velocity of the composite particle is $\frac{V}{3}$.