Explanation

(b) Here to point 7 of problem Solving skills
$\lbrack\frac{h_{1}}{h_{2}} = \frac{g_{2}}{g_{1}}or\ h_{2} = \frac{h_{1}g_{1}}{g_{2}} = \frac{0.5 \times g}{g/6} = 3.0\rbrack$
Energy spent = $mg\ h_{e} = mg_{m}h_{m}$
or $h_{m} = g_{e}h_{e}/g_{m}\ $ …(i)
$= \left( \frac{G\frac{4}{3}\pi R_{e}^{3}\rho/R_{e}^{2})h_{e}}{\left( G\frac{4}{3}\pi R_{m}^{2}\rho_{m}/R_{m}^{2} \right)} \right) = \left( \frac{G\frac{4}{3}\pi R_{e}^{3}\rho/R_{e}^{2})h_{e}}{\left( G\frac{4}{3}\pi R_{m}^{2}\rho_{m}/R_{m}^{2} \right)} \right)$
$= \frac{R_{e}}{R_{m}} \times \frac{\rho_{e}}{\rho_{m}} \times h_{e} = \frac{3}{2} \times \frac{4}{1} \times 0.5 = 3m$