When a polycrystal is immersed in a liquid which is in chemical equilibrium with the solid, the liquid can penetrate along the grain boundaries according to the dihedral angle and form a solid-liquid two-phase microstructure. However, if the boundary energy is not constant but varies with grain boundary orientation, some boundaries with low energies may not be penetrated by the liquid even though the apparent dihedral angle is 0. Assuming the nearest neighbour grains on a two-dimensional plane to be six, a researcher estimated the fraction of the unpenetrated grain boundary area in three dimemsions to be (number of grain boundaries/ number of grains × 6) × 100(%). Discuss the validity of his estimation.