
$F = m \omega^{2} A =\mu mg$
So,
$A =\frac{\mu g }{\omega^{2}}$ And $\omega=\sqrt{\frac{ K }{ m }}$
So,
$\omega=\sqrt{\frac{100}{1.5}}$
Substitute the values.
$A =\frac{(0.4)(9.8)}{\left(\sqrt{\frac{100}{1.5}}\right)^{2}}$
$=6\, cm$



($A$) The amplitude of oscillation in the first case changes by a factor of $\sqrt{\frac{M}{m+M}}$, whereas in the second case it remains unchanged
($B$) The final time period of oscillation in both the cases is same
($C$) The total energy decreases in both the cases
($D$) The instantaneous speed at $x_0$ of the combined masses decreases in both the cases