As no external force is acting on the bullet-gun system, the momentum of the system will remain conserved.
Let m1= Mass of the bullet = 60 g = 0.06 kg
m2= Mass of the gun = 10 kg
v1 = Velocity of the bullet = 60 m/s
v2 = Recoil velocity of the gun
Applying the conservation of momentum, m1v1=m2v2
⇒0.06×60=10×v2
⇒v2=0.36 m/s