Problem 4. Let SABCD be a pyramid, such that the base ABCD is a parallelogram. Let A' be the point on SA such that SA = 5SA', let B' be the point on SB such that SB = 4SB', and let C' be the point on SC such that SC 3SC;. Let D' be the point of intersection of the plane ABC and the segment SD. Find the ratio SD SD'. You will need to use the following fact: if you place masses at the points A', B', C', then their center of mass will lie in the plane ABC. =