Two blocks of masses $10 \mathrm{kg}$ and $30 \mathrm{kg}$ are placed on the same straight line with coordinates $(0,0)\mathrm{cm}$ and $(x,0)\mathrm{cm}$ respectively. The block of $10 \mathrm{kg}$ is moved on the same line through a distance of $6 \mathrm{cm}$ towards the other block. The distance through which the block of $30 \mathrm{kg}$ must be moved to keep the position of centre of mass of the system unchanged is :