$\begin{aligned} \frac{1}{C_{e q}} & =\frac{1}{C_1}+\frac{1}{C_2} \\ & =\frac{1}{K \epsilon_0 A}+\frac{1}{\epsilon_0 A} \\ & =\frac{1}{K \epsilon_0 A}+\frac{d-t}{\epsilon_0 A} \\ & =\frac{1 \times 10^{-3}}{5 \epsilon_0 \times 40 \times 10^{-4}}+\frac{1 \times 10^{-3}}{\epsilon_0 40 \times 10^{-4}} \\ \frac{1}{C_{e q}} & =\frac{1}{20 \epsilon_0}+\frac{1}{4 \epsilon_0} \\ C_{e q} & =\frac{20 \times 4 \epsilon_0}{24}=\frac{10 \epsilon_0}{3} \mathrm{~F}\end{aligned}$