A man fires a bullet of mass 200g at a speed of  $\text{5m}/\text{s}$. The gun is of one kg mass. By what velocity the gun rebounds backward?

Given, mass of bullet,
${\text{m}}_{\text{b}}=200\text{\hspace{0.17em}g}$
$= 0.2 kg$
Velocity of bullet,
${\text{v}}_{\text{b}}=5\text{\hspace{0.17em}m}/\text{s}$
Mass of gun 
${\text{m}}_{\text{g}}=1\text{\hspace{0.17em}kg}$
The recoil velocity of the gun,
$\text{v}=\frac{{\text{m}}_{\text{b}}{\text{v}}_{\text{b}}}{{\text{m}}_{\text{g}}}=\frac{0.2\times 5}{1}=1\text{m}/\text{s}$