Answer
$12.4\%$
Work Step by Step
For copper, use the same formula to find volume per cubic meter
$V_{Cu} = \frac{m N_A(4/3 \pi r^3)}{M_{Cu}} $
Where
$m = 8960kg$
$N_A$ is $6.022 \times 10^{23}/mol$
$r = 135.0 \times 10^{-12}m$
$M_Cu = 63.5g/mol$
So,
$V_{Cu} = \frac{(8960 kg)(6.022 \times 10^{23}/mol)4/3 \pi (135.0 \times 10^{-12}m)^3}{63.5g/mol} $
$V_{Cu} = 0.1876m^3$
Fraction available for conduction is
$1-(V_{Cu}/1.00 m^3)$
$ = 1- 0.1876 = 0.124 $
$ = 12.4\%$
