The ratio of the equilibrium number of vacancies given in the problem statement (Nv= 2.3 x 10^25m^3) and the value of N is obtained using this formula N=NAp/A .
Thus;
The fraction of vacancies is equal to the Nv/Nratio.
Firstly the value of N is obtained with N=NAp/A
=( (6.022 x 10^23 atoms/mol)(5.60 g/cm3 )(10^6cm^3/m^3) )/65.6 g/mol