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