Let $\left[{\epsilon }_0\right]$ denote the dimensional formula of the permittivity of vacuum. If M=mass, L=length, T=time and A=electric current, then:

Density  $\rho =\frac{M}{L^3}$
 $n_1{u}_1=n_2{u}_2$         
$\therefore 128\left[M_1{L}_1^{-3}\right]=n_2\left[M_2{L}_2^{-3}\right]$ 
$\therefore n_2=128\left[\frac{M_1}{M_2}\right]{\left[\frac{L_2}{L_1}\right]}^3=128\times \left[\frac{1000}{50}\right]\times {\left[\frac{25}{100}\right]}^3=128\times 20\times \frac{1}{64}=40$