A bullet moving with a speed of $400 \mathrm{m} {\mathrm{s}}^{-1}$ penetrates a sand bag and drops dead inside it. The masses of the bullet and the sand bag are $0.25 \mathrm{kg}$ and $4.75 \mathrm{kg}$, respectively. If the bag is free to move, its velocity shall be (in $\mathrm{m} {\mathrm{s}}^{-1}$)

A block of mass $m$ rests on a rough horizontal surface (Coefficient of friction is $\mu$). When a bullet of mass $\frac{m}{2}$ strikes horizontally, and get embedded in it, the block moves a distance $d$ before coming to rest. The initial velocity of the bullet is $k\sqrt{2\mu gd}$, then the value of $k$ is