A stone is dropped at t - 0. A second stone, with twice the mass of the

Question:

A stone is dropped at t - 0. A second stone, with twice the mass of the first, is dropped from the same point at t = 100ms.
(a) How far below the release point is the center of mass of the two stones at t = 300 ms? (Neither stone has yet reached the ground.)
(b) How fast is the center of mass of the two-stone system moving at that time?