Frankel defect does not put any effect on the density of solids.

Value of Z is 4 as the crystal has FCC lattice. Density of the crystal is determined as:

$\begin{array}{ccc}\mathrm{D}& =& \frac{\mathrm{MZ}}{{\mathrm{a}}^3\times {\mathrm{N}}_{\circ }}\\ \mathrm{D}& =& \frac{40\times 4}{{\left(0.556\times {10}^{-7}\right)}^3\times 6.02\times {10}^{23}}\\ \mathrm{D}& =& 1.546 \mathrm{g}/{\mathrm{cm}}^3\end{array}$