A body of mass $m_1$ moving with an unknown velocity of ${\mathrm{v}}_1\hat{i}$, undergoes a collinear collision with a body of mass $m_2$ moving with a velocity  ${\mathrm{v}}_2\hat{i}$ . After collision, m₁ and m₂ move with velocities of  ${\mathrm{v}}_3\hat{i}$ and ${\mathrm{v}}_4\hat{i}$ , respectively. If m₂ = 0.5 m₁ and v₃ = 0.5 v₁, then v₁ is :