The particle with velocity 2v and mass m1 to P by AQP and will rotate 240° (or 4π∕3) while the particle with velocity v and mass m2 reaches P from A by rotating 120° (or 2π∕3). At P, they make elastic collision, then m2 moves back to Q by PAQ as it has now velocity 2v and m1 come back from from P to Q to make collision once more. Now, m1 has velocity 2v and m2 has velocity v. So, the particles reach A after two collisions as shown in figure below