In this video you are shown what we mean by the centre of mass as I demonstrate with a set of particles in a line.

You are then asked:

Find the centre of mass of three particles of masses 4kg, 5kg and 6kg placed on the x-axis at the points (3,0), (6,0) and (13,0) respectively.

Centres of Mass : Particles in a line : ExamSolutions - youtube Video