When a dielectric slab of dielectric constant K = 23 is inserted between the plates of C2 , its new capacitance (C2′) become C2′ = 23 C Equivalent capacitance of C2′ and C3 is Ceq = C2′+C3 = 23C + C = 25C
Now, Ceq and C1 are in series. Therefore, their equivalent capacitance is Ceq = Ceq+C1Ceq×C1 = 25C+C2×25C×C = 7C5C2 = 75C