For a capacitor, whatever insulator goes between conductor plates, is a dielectric. If one dielectric material fills up half the thickness between plates, the other half is air, and you have a mixed dielectric capacitor.
You can calculate the capacitance value by considering two capacitors in series, each having the thickness of individual dielectric thickness. Watch these two videos to learn more...
Part-1Part-2
If you are interested in going directly through the solutions of Chapter: Capacitor and Dielectrics. Kindly CLICK HERE...