How many atoms are present per cubic centimeter of W?
Since it is a BCC cell, then it will have 4 atoms per unit cell.
As a solid, W adopts a body-centered cubic unit cell. How many unit cells are present per cubic centimeter of W?
(19.3 g/cm^3)( 1 mol / 183.84 g) ( 6.022x10^23 atoms / 1 mol ) ( 1 cell / 4 atoms) = 1.58x10^22 cells / cm^3
Hope this helps.