$($a$)$ Calculate the fraction of atom sites that are vacant for copper $(Cu)$ at a temperature of $488C(761K).$ Assume an energy for vacancy formation of $0\mathrm{.}90e\frac{V}{?}$ atom.

$($b$)$ Repeat this calculation at room temperature $(298$ K$).$

$($c$)$ What is ratio of $\frac{N_v}{N}(761K)$ and $\frac{N_v}{N}(298K)?$