A parallel plate capacitor is of area $6c{m}^2$  and a separation $3 mm$. The gap is filled with three dielectric materials of equal thickness (see figure) with dielectric constants  $K_1=10$, $K_2=12$   and  $K_3=14$. The dielectric constant of a material which when fully inserted in above capacitor, gives equivalent  capacitance would be :