What is the minimum limiting radius ratio for an octahedral arrangement?

$A$ solid has a structure in which $W$ atoms are located at the corners of a cubic lattice,$O$ atoms at the centre of edges,and $Na$ atom at the centre of the cube. The formula for the compound is

The vacant space in the $bcc$ unit cell is .............. $\%$

Which of the following is correct for the crystal lattices mentioned below?

The coordination number of a metal crystallizing in a hexagonal close-packed $(hcp)$ structure is:

If $a$ is the length of the side of a cube,the distance between the body-centered atom and one corner atom in the cube will be