${\mathrm{Z}}_{\mathrm{eff}}$ of 3s electron in Zn is:

$\begin{array}{l}\left(1s^2\right)\left(2s^{2}2p^6\right)\left(3s^{2}3p^6\right)\left(3d^{10}\right)\left(4s^2\right)\\ \sigma =(2\times 1)+(8\times 0.85)+(7\times 0.35)\\ \sigma =2+6.8+2.45\\ \sigma =11.25\end{array}$

$\begin{array}{r}\text{ Hence, }{\mathrm{Z}}_{\text{eff }}=\mathrm{Z}-\sigma \\ =30-11.25=18.75\end{array}$