A heavier sphere moving eastward with a certain velocity 'v' collides with a lighter sphere at rest. If it is perfect elastic head on collision,
then after collision 

As the collision is elastic the velocity of approach is equal to velocity of separation.

Even after collision the heavier sphere will continue to move with the same velocity v.

Let the velocity of small sphere after collision be v'.

v'-v=v$\Rightarrow$v'=2v