Two particles $A$ and $B$ of masses $20 \mathrm{g}$ and $30 \mathrm{g}$, respectively are at rest at a certain time. Because of the forces exerted by them on each other, the particles start moving. At a given instant, particle $A$ is found to move towards the East with a velocity of $6 \mathrm{c}\mathrm{m} {\mathrm{s}}^{-1}$. What is the velocity of particle $B$ at this instant?