Respuesta :

Hagrid
The formula for the theoretical density of a solid crystal is
d = n M / V A
where 
d is the density
M is the molar mass
V is the volume of the cell
A is the Avogadro's number
Subsituting
7.87 = 1 (56) / (2r√2)³ (6.022x10^23)
Solve for r