A disc of mass $1.0 \mathrm{kg}$ is kept floating horizontally in air by firing bullets of mass $0.05 \mathrm{kg}$ each vertically at it, at the rate of $10$ per second. If the bullets rebound with the same speed, the speed (in m/s) with which these are fired will be…………..  $\left[g=9.8 \raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$s^2$}\right.\right]$