1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution The direction of motion of all the three particles are changing w.r.t. lab frame. But , their relative velocity remains constant. relative velocity = vb - va And the distance between the two particles decreases from a to 0. So, t = a/|vb - va| I am not clear why I am taking this step. I just feel tempted to take it. |vb - va| = (√3)v t = a/(√3)v W.r.t. A's frame of reference , B is always moving at an angle 60° to the line joining A and B. Then, how can the two ever meet? After seeing the answer (which is 2a/3v)in the book, Due to the symmetry of the problem, the three will converge at the centre of the triangle. The distance between the center and the vertices is a/√3. This is the magnitude of the displacement traveled by each point. So, taking t = (a/√3)/ [|vb - va| = (√3)v] = a/3v Even now, I am missing a factor of 2. I know that the relative velocity is constant and the displacement of each particle but I can't connect the two information.