The density of case of a planet is $s_1$ and that of the outer shell is $s_2$.The radius of case and that of the planet are R and 2R respectively. Gravitational acceleration at the surface of the planet is same as at a depth R. Find $\large\frac{s_1}{s_2}$

Question

$\begin{array}{1 1} \large\frac{1}{3} \\\large\frac{7}{3} \\ \large\frac{4}{3} \\ \large\frac{8}{3} \end{array}$