Question

a bullet of mass 10g is fired at a speed 500m/s from the gun of mass 5kg. what is the recoil velocity of the gun

Answer 1

let m= mass of bullet
and M= mass of gun

V= RECOIL VELOCITY and v= velocity of bullet

since initially the gun was at rest , there initial momentum = 0
therefore the final momentum should also remain zero 
i.e
mv +MV = O 
therefore V= -mv/M = .01*500/5 = -1m/s*2

Answer 2

here is your answer