a
(a) \(g = \frac{4}{3}\pi G\rho R\) \( \Rightarrow \,\,g \propto \rho R\, \Rightarrow \,\frac{{{g_e}}}{{{g_m}}} = \frac{{{\rho _e}}}{{{\rho _m}}} \times \frac{{{R_e}}}{{{R_m}}}\)
\( \Rightarrow \,\,\frac{6}{1} = \frac{5}{3} \times \frac{{{R_e}}}{{{R_m}}}\, \Rightarrow \,\,{R_m} = \frac{5}{{18}}{R_e}\)