The capacitance of an air filled parallel plate capacitor is $10 \times 10^{-12}$F. The separation between the plates is doubled and the space between the plates is then filled with wax giving the capacitance a new value of 40$\times$10^-12 farads. The dielectric constant of wax is   

8

$ C = \frac{\epsilon_0 A}{d} = 10\times 10^{-12} F$

After changing the distance and filling the space with wax the capacitance becomes

$ C'= \frac{k\epsilon_0 A}{2d} = 40\times 10^{-12} F $

$ \Rightarrow k = 8$