whenever a new mass is added to spring the equilibrium state of the spring will change and then the mas will start oscillating about the new equilibrium position.
So here we can say that mass will always oscillate about its new equilibrium position if it is changed
Now in next situation it is given that
spring constant(k) = 4.9 kg/s^2
displacement of mass after suspending through spring = 20 cm
now we have the formula of spring force as
here applied force is due to gravity so we have
so mass of object will be 100 g or 0.10 kg