d
\(I = {I_1} + {I_2} + {I_3} + {I_4} + .......\) \( = \frac{{GM}}{{r_1^2}} + \frac{{GM}}{{r_2^2}} + \frac{{GM}}{{r_3^2}} + \frac{{GM}}{{r_4^2}} + ..........\)

\( = GM\,\left[ {\frac{1}{{{1^2}}} + \frac{1}{{{2^2}}} + \frac{1}{{{4^2}}} + \frac{1}{{{8^2}}} + .......} \right]\)

\( = GM\left[ {1 + \frac{1}{4} + \frac{1}{{16}} + \frac{1}{{64}} + .........} \right]\)

\( = GM\,\left( {\frac{1}{{1 - \frac{1}{4}}}} \right)\)      [\( = \frac{a}{{1 - r}}\)]

\( = GM \times \frac{4}{3}\)

\( = G\, \times 3 \times \frac{4}{3} = 4G\)    [\(M = 3kg\) ]