a
First case $\frac{2}{40}=\frac{X}{60} \Rightarrow X=3 \Omega$
In second case $X^{\prime}=\frac{2 \times 3}{2+3}=1.2 \Omega$
$\frac{2}{\ell}=\frac{1.2}{100-\ell}$
$200-2 \ell=1.2 \ell$
$\ell=\frac{200}{3.2}=62 . \mathrm{bm}$
Balance length changes by $22.5 \mathrm{~cm}$
