A body of mass $m_1$ moving with an unknown velocity of $v_1 \hat{i}$, undergoes a collinear collision with a body of mass $\mathrm{m}_2$ moving with a velocity $\mathrm{v}_2 \hat{\mathrm{i}}$. After collision, $\mathrm{m}_1$ and $\mathrm{m}_2$ move with velocities of $\mathrm{v}_3 \hat{\mathrm{i}}$ and $\mathrm{v}_4 \hat{\mathrm{i}}$, respectively. If $\mathrm{m}_2 =0.5 \mathrm{~m}_1$ and $\mathrm{v}_3=0.5 \mathrm{v}_1$, then $\mathrm{v}_1$ is :