Consider a large block placed on a smooth horizontal surface, with a man standing at one end of the block. The man walks to the other end, relative to the block. The distances (absolute) moved by the man and the block are:

Let m and M be the respective masses of the person and the block, and let l be its length. Now, if x1 and x2 be the absolute displacement of the person and the block and x that of the centre of mass, then x = mx1+mx2m+M ⇒∆x = m∆x1+m∆x2m+M Since horizontal forces are absent, so  ∆x = 0m∆x1+M∆x2 = 0    ⇒ m(-l)+(m+M)∆x2 = 0∆x2 = mlm+MSimilarly, ∆x1 = Mlm+M