In hexagonal systems of crystals, a frequently encountered arrangement of atoms is described as a hexagonal prism.Here, the top and bottom of the cell are regular hexagons and three atoms are sandwiched in between them.A space-fitting model of this structure, called hexagonal closed-packed (HCP), is constitution of a sphere on a flat surface surrounded in the same plane by six identical spheres as closely as possible Three spheres are then placed over the first layer so that they touch each other and represent the second layer.Each one of these three spheres touches three spheres of the bottom layer.Finally, the second layer is covered with third layer that is identical to the bottom layer in relative position.Assume radius of even sphere to be 'r'. In hcp lattice, if a plane is drawn parallel to layers A and B at a distance c/8 above the layer B, it passes through centres of x tetrahedral and y octahedral voids.A plane parallel to layer A at a distance c/8 just below it passes through centres of p tetrahedral and q octahedral voids.Find (x+y+p+q) c is the cell parameter of hcp lattice, the vertical edge length.