
With insertion of dielectric, the system can be considered as combination of two capacitors in series.
Thus, equivalent capacitance of the system is
Ceq1=C11+C21
=tKϵ0A1+d−tϵ0A1
=Kϵ0At+ϵ0Ad−t
=5ϵ0×40×10−41×10−3+ϵ0×40×10−41×10−3
⇒Ceq1=20ϵ01+4ϵ01
⇒Ceq=2420×4ϵ0=310ϵ0F.