\(\mathrm{Q}=\frac{\mathrm{A} \in_0}{\mathrm{~d}} \mathrm{~V}\)

with dielectric

Q=\(\frac{A \in_0 V}{d-t+\frac{t}{K}}\)

given

\(\frac{\mathrm{A} \in_0 \mathrm{~V}}{\mathrm{~d}-\mathrm{t}+\frac{\mathrm{t}}{\mathrm{K}}}=(1.25) \frac{\mathrm{A} \in_0 \mathrm{~V}}{\mathrm{~d}}\)

\(\Rightarrow 1.25\left(3+\frac{2}{\mathrm{~K}}\right)=5\)

\(\Rightarrow \mathrm{K}=2\)