Answer
$\frac{R_A}{R_B} = 3.0$
Work Step by Step
We can find the cross-sectional area of conductor A:
$A_A = \pi~(0.50~mm)^2 = (0.25~\pi)~mm^2$
We can find the cross-sectional area of conductor B:
$A_B = \pi~[(1.0~mm)^2-(0.50~mm)^2]$
$A_B = (0.75~\pi)~mm^2$
We can find the ratio $\frac{R_A}{R_B}$:
$\frac{R_A}{R_B} = \frac{\rho~L/A_A}{\rho~L/A_B}$
$\frac{R_A}{R_B} = \frac{A_B}{A_A}$
$\frac{R_A}{R_B} = \frac{(0.75~\pi)~mm^2}{(0.25~\pi)~mm^2}$
$\frac{R_A}{R_B} = 3.0$
