a
(a) In balancing condition, \(\frac{{{R_1}}}{{{R_2}}} = \frac{{{l_1}}}{{{l_2}}} = \frac{{{l_1}}}{{100 - {l_1}}}\)
\( \Rightarrow \) \(\frac{X}{Y} = \frac{{20}}{{80}} = \frac{1}{4}\).....\((i)\)
and \(\frac{{4X}}{Y} = \frac{l}{{100 - l}}\).....\((ii)\)
\( \Rightarrow \) \(\frac{4}{4} = \frac{l}{{100 - l}}\)\( \Rightarrow \) \(l = 50\,cm\)