Original resistance: [ R = \rho \frac{L}{A} = 20 \ \Omega ]

[ A \times L = A' \times L' ] [ A \times L = A' \times (3L) ] [ A' = \frac{A}{3} ]

New resistance: [ R' = \rho \frac{L'}{A'} = \rho \frac{3L}{A/3} ] [ R' = \rho \frac{3L \times 3}{A} = \rho \frac{9L}{A} ] [ R' = 9 \left( \rho \frac{L}{A} \right) ] [ R' = 9 \times R ]