A parallel plate condenser with plate area A and separation d filled with dielectric as shown in fig. The dielectric constants are K1 and K2 respectively. The capacitors will be

The combination is equivalent to two capacitors in series, each with plate area A and separation d/2.C1=K1ε0A(d/2)=2K1ε0AdC2=K2ε0Ad/2=2K2ε0AdFurther 1C=1C1+1C2Substitute the values of C1 and C2 and obtain C.