[permittivity of free space $ = 9 \times 10^{-12}\ Fm^{-1}$]
$\mathrm{V}=20 \times 10^{6} \times 40 \times 10^{-6}=800$
$Q_{\max }=C V=\frac{K \epsilon_{0} A}{d} V$
$ = \frac{{4 \times 5 \times {{10}^{ - 12}} \times 6400 \times {{10}^{ - 6}}}}{{40 \times {{10}^{ - 6}}}} \times 800$
$=4608 \times 10^{-9} \mathrm{\,C}$



