If the de-Broglie wavelength of the electron in nth Bohr orbit in a hydrogen atom is equal to  $1.5\pi a_0$ ( is Bohr radius), then the value of n/Z is  

Number of waves $=\frac{circumference}{Wavelength}$

$n=\frac{2\pi r}{\lambda }\Rightarrow 2\pi r=n\lambda \mathrm{....}\left(i\right)$

Also, we know that radius (r) of an tom is given by  $r=\frac{a_0{n}^2}{Z}$

Thus, Eq. (i) becomes 

$2\pi a_0\frac{n_2}{Z}=n\lambda \mathrm{.......}\left(ii\right)$

$\therefore 2\pi a_0\frac{n^2}{Z}=n\left(1.5\pi a_0\right)$

$\left(Given,\lambda =1.5\pi a_0\right)$

$\frac{n}{Z}=\frac{1.5\pi a_0}{2\pi a_0}=\frac{1.5}{2}=0.75$