A ball travelling at 6 m/s collides with another ball twice its mass.The first ball due to the collision moves at 3 m/s in the reverses direction....

Let M be the mass of the first ball , which collides with
the second ball of mass 2M.

The velocity of the first ball
is 6m/s.

The second ball's velocity is zero as the ball is
asumed to be at rest.

We presume the first ball  moves with
the a certain  velocity v1 (not 3m/s as given in the problem) after
collision.

Assuming the elastic collision, the velocities
of the first and second ball after collision should be as
below:

v1 ={ (M-2M)/(M+2M}6m/s = - 2m/s. (negative
indicates the velocity is in the reverse direction.

v2 =
2M*6/(m+2M) = 4m/s.

Therefore , the second ball moves with
a velocity 4 m/s after collision.

There is an internal
inconsistency in the given problem. How can a ball reverse its  velocity at 3m/s  which
is higher than the velocity after a perfect elastic collision . Normally the energy is
dissipated  even in perfect collision.

Hope this helps an
inquisitive mind.