At 20^oC the resistance of a copper wire of length 30m and diameter o.1 cm is $0.65\Omega$.The resistivity of copper wire at 20^oC is 

$1.702 \times 10^{-8} \Omega - m$

$\rho = \frac{RA}{L} $

$ A = \frac{\pi d^2}{4} = \frac{\pi 10^{-6}}{4} = 0.78\times 10^{-6}$

$ \rho = \frac{0.65 \times 0.78\times 10^{-6}}{30} = 1.702\times 10^{-8} $