If one ball with a momentum of 50 kg m/s hits a ball at rest an no other forces are acting on either ball, what's the total momentum of both balls after the collision?

The total momentum of both ball=Momentum of moving ball+momentum of ball at rest

Because the ball is at rest, there would be no velocity and if no velocity than momentum of object at rest will be=0

therefore total momentum = 50+0

=50 Kg m/s