In the given circuit the the internal resistance of the 18 V cell is negligible. If R₁ = 400 $$\Omega $$, R₃ = 100 $$\Omega $$ and R₄ = 500 $$\Omega $$ and the reading of an ideal voltmeter across R₄ is 5V, then the value of R₂ will be :

A parallel plate capacitor with square plates is filled with four dielecytrics of dielectrics constants K₁, K₂, K₃, K₄ arranged as shown in the figure. The effective dielectric constant K will be :