c
For first balancing condition $\frac{{10 + {R_1}}}{{{R_2}}} = \frac{{50}}{{50}}$
$ \Rightarrow $ ${R_2} = 10 + {R_1}$.
For second balancing condition
$\frac{{{R_1}}}{{{R_2}}} = \frac{{40}}{{60}}$ $ \Rightarrow $ $\frac{{{R_1}}}{{10 + {R_1}}} = \frac{2}{3}$
$ \Rightarrow $ ${R_1} = 20\,\Omega $