Particles P and Q are resting on a smooth, horizontal surface. A smooth, vertical wall is at a distance 2r from Q, and the wall is perpendicular to the line through P and Q. The masses of P and Q are 2m and m, respectively. The coefficient of restitution between P and Q is 2/3, and the coefficient of restitution between Q and the wall is 2/5. P is projected towards Q with velocity u. Find the distance of the particles from the wall when they collide for a second time.