$A = \pi {R^2} = \pi \frac{{{d^2}}}{4} \Rightarrow \frac{{{A_A}}}{{{A_B}}} = {(\frac{{{d_A}}}{{{d_B}}})^2}$$R = \rho \frac{L}{A} \Rightarrow \frac{{{R_B}}}{{{R_A}}} = \frac{{{L_B}}}{{{L_A}}} \times \frac{{{A_A}}}{{{A_B}}} = \frac{{{L_B}}}{{{L_A}}}{(\frac{{{d_A}}}{{{d_B}}})^2}$$\frac{{{R_B}}}{{{R_A}}} = \frac{{{L_B}}}{{4{L_B}}}{(\frac{{4{d_B}}}{{{d_B}}})^2} = 4$$\frac{{{R_B}}}{5} = 4 \Rightarrow {R_B} = 20\Omega $