$\frac{R}{S}=\frac{25}{75}$

New resistance $\mathrm{R}^{\prime}=\frac{\rho \ell}{\mathrm{A}}$

$=\frac{\rho \times \frac{\ell}{2}}{\frac{\mathrm{A}}{4}}=\frac{\rho \ell}{2} \times 4 \mathrm{A}$

$\mathrm{R}^{\prime}=2 \mathrm{R}$

$\frac{2 \mathrm{R}}{\mathrm{S}}=\frac{\ell}{100-\ell^{\prime}}$

$2 \times \frac{1}{3}=\frac{\ell^{\prime}}{100-\ell^{\prime}}=3 \ell^{\prime}=200-2 \ell^{\prime}$

$5 \ell '=200$

$\ell^{\prime}=40$