a
સંતુલન સ્થિતિમાં \(\frac{{{R_1}}}{{{R_2}}} = \frac{{{{l}_1}}}{{{{l}_2}}} = \frac{{{{l}_1}}}{{100 - {{l}_1}}}\,\)

\( \Rightarrow \,\,\frac{X}{Y} = \frac{{20}}{{80}} = \frac{1}{4}\,\,\,\,\,\,........\,\,(i)\,\,\)

\( \Rightarrow \,\,\,\frac{{4X}}{Y} = \frac{{l}}{{100 - {l}}}\,\,\,\,\,........\,\,(ii)\,\,\,\)

\(\,\frac{4}{4} = \frac{{l}}{{100 - {l}}}\,\, \Rightarrow \,\,l = 50\,cm\)