\(\lambda = \frac{{2\pi r}}{n} = \frac{{2\pi {n^2}{a_0}}}{{n \times Z}} = 2\pi \frac{n}{Z}{a_0}\)
\(\lambda = 1.5\pi {a_0}\)
\(\therefore \,2\pi \frac{n}{Z}{a_0} = 1.5\pi {a_0}\)
\(\therefore \frac{n}{Z} = \frac{{1.5}}{2} = 0.75\)