How many spheres form an octahedral void in a close-packed structure?

$A$ solid element (metal) crystallises in $ABABAB$ type packing in three dimensions. If the density of the element and its atomic radius are $5.0 \ g/cm^3$ and $100\sqrt{2} \ pm$ respectively, the atomic mass of the element is $(N_A = 6 \times 10^{23} \ mol^{-1})$

The fraction of total volume occupied by the atoms present in a simple cubic unit cell is

Which of the following metals crystallizes in a hexagonal close-packed $(hcp)$ structure?

In crystalline silicon,$Si$ atoms occupy all the $ccp$ sites and every alternate tetrahedral void. The value of packing efficiency is closest to $....\%$

In which of the following crystals are all the tetrahedral voids occupied?