How many tetrahedral voids are occupied in diamond? (in percentage)

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-filling model of this structure,called hexagonal close-packed $(HCP)$,is constituted 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 a third layer that is identical to the bottom layer in relative position. Assume radius of every sphere to be $r$.
$1.$ The number of atoms on this $HCP$ unit cell is
$(A)$ $4$ $(B)$ $6$ $(C)$ $12$ $(D)$ $17$
$2.$ The volume of this $HCP$ unit cell is
$(A)$ $24 \sqrt{2} r^3$ $(B)$ $16 \sqrt{2} r^3$
$(C)$ $12 \sqrt{2} r^3$ $(D)$ $\frac{64 r^3}{3 \sqrt{3}}$
$3.$ The empty space in this $HCP$ unit cell is
$(A)$ $74 \%$ $(B)$ $47.6 \%$ $(C)$ $32 \%$ $(D)$ $26 \%$
Give the answer for questions $1, 2$ and $3.$

In a crystal lattice,the number of octahedral voids compared to tetrahedral voids is ...............

In a face-centred cubic $(FCC)$ lattice,where are the tetrahedral voids located?

$A$ solid compound is formed by atoms of $A$ (cations),$B$ (cations),and $O$ (anions). Atoms of $O$ form an $hcp$ lattice. Atoms of $A$ occupy $25\%$ of the tetrahedral voids,and atoms of $B$ occupy $50\%$ of the octahedral voids. What is the molecular formula of the solid?

Write a short note on: Formula of a compound and number of voids filled.