Two particles of mass $1 \mathrm{kg}$ and $3 \mathrm{kg}$ move towards each other under their mutual force of attraction. No other force acts on them. When the relative velocity of approach of two particles is $2 \mathrm{m} {\mathrm{s}}^{-1}$, their centre of mass has a velocity of $0.5 \mathrm{m} {\mathrm{s}}^{-1}$. When the relative velocity of approach becomes $3 \mathrm{m} {\mathrm{s}}^{-1}$, the velocity of the centre of mass will be