Bullet's speed when separated from the gun is 200 m/s. if the mass of the bullet and the gun are 10 g and 5 kg, what is speed of gun?

mass of bullet =mb= 10g = 0.01 kg
mass of gun =mg= 5 kg
v gun = 200m/s
speed of gun before shot fired= vg = 0
speed of bullet leaving the gun = vb = 0 

[tex]m_g v_g + m_b v_b = m_g v'_g + m_b v'_b[/tex]
[tex]5 .0 + 0,01 . 0 = 5 v'_g + 0,01 .200[/tex]
[tex]0+0= 5 . v_g + 2[/tex]
[tex]-2= 5 v_g[/tex]
[tex]v_g=-0.4[/tex]m/s

minus sign (-) means that the gun is moving backwards