Two persons of masses 55 kg and 65 kg respectively, are at the opposite ends of a boat. The length of the boat is 3.0 m and weight 100 kg. The 55 kg man walks up to the 65 kg man and sits with him. If the boat is in still water the centre of mass of the system shifts by:

0

Net external force is equal to zero so there is no displacement of centre of mass of the system. Thus, centre of mass remains unchanged.