A stone of mass $500 \mathrm{g}$ is dropped from the top of a tower of $100 \mathrm{m}$ height. Simultaneously another stone of mass $1 \mathrm{kg}$ is thrown horizontally with a speed of $10 \mathrm{m} {\mathrm{s}}^{-1}$ from same point. The height of the centre of mass of the above two stone system after $3 \sec$ is $\left(g=10 \mathrm{m} {\mathrm{s}}^{-2}\right)$