two identical air core capacitors are connected in series to a voltage source of 15V.If one of the capacior is filled with a medium of dielectric constant 4 .what is the potential across this capacitor

By inserting the dielectric, the capacitance of the capacitor becomes 4 times of that capacitor in which dielectric is not added. When capacitors are connected in series, the voltage drops across them in inverse ratio of their capacitances.
Hence the voltage across the capacitor in which the dielectric has been added is