A parallel plate capacitor is made up of stair like structure with...

Here the system can be considered as three capacitors in parallel.

So, net capacitance is

$C = \dfrac{{ε}_{0}A}{5b} + \dfrac{{ε}_{0}A}{3b} + \dfrac{{ε}_{0}A}{b}$

$C = \dfrac{{ε}_{0}A}{b}\left[\dfrac{1}{5} + \dfrac{1}{3} + \dfrac{1}{1}\right]$

$C = \dfrac{23{ε}_{0}A}{15b}$