If thickness of the copper slab is $l$, the separation of plates between will be $\frac{1}{2}(d-l)$ The capacitance before insert slab is $C=\frac{A \epsilon_{0}}{d}$
The capacitance of each capacitor after insert slab is $C^{\prime}=\frac{A \epsilon_{0}}{(d-l) / 2}$
Net capacitance after insert slab is $C_{e q}=\frac{C^{\prime} C^{\prime}}{C^{\prime}+C^{\prime}}=\frac{c^{\prime}}{2}=\frac{A \epsilon_{0}}{(d-l)}$
thus, $C_{e q}>C$ and if sheet is very thin $(d>>l), C_{e q}=C$


[Take $\ln 2=0.69]$

