Eight wire in parallel, then equivalent resistance is $\frac{ r }{8}=\frac{\rho \ell}{2 \pi d ^{2}}$

Single copper wire of length $2 l$ has resistance

$R =\rho \frac{2 \ell \times 4}{\pi d _{1}^{2}}=\frac{\rho \ell}{2 \pi d ^{2}}$

$d _{1}=4 d$