$$
\begin{aligned}
& \frac{25}{\mathrm{r} \ell_1}=\frac{\mathrm{X}}{\mathrm{r} \ell_2} \\
& \frac{25}{2 \mathrm{r} \ell^{\prime}}=\frac{\mathrm{X}}{2 \mathrm{r} \ell^{\prime}}
\end{aligned}
$$
From (i) and (ii)
$$
\ell_2^{\prime}=\ell_2=40 \mathrm{~cm}
$$