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 \( \frac{V}{3} \).