An object of mass  m1 collides with another object of mass  m2, which is at rest. After the collision the objects move with equal speeds in opposite direction. The ratio of the masses m2:m1  is

Before collision,  u1=u;u2=0,  After collision v1=−v;v2=v  If the collision is elastic, then we can write  u1−u2=v2−v1 ⇒u=2v   -----(1)Applying principle of conservation of momentum  m1u=m2−m1v -----2From (1) & (2) we get  m2m1=3