Two blocks of masses $m$ and $2m$ are placed on a smooth horizontal surface and connected by a spring. If an impulse $J$ is applied to the lighter block, the velocity of the center of mass immediately after the impulse is:

Two bodies of masses 5 kg and 3 kg are moving towards each other with $2\,m{s^{ - 1}}$ and $4\,m{s^{ - 1}}$ respectively. Then velocity of centre of mass is