The molar ratio of $Fe^{2+} to\; Fe^{3+} in\; a\; mixture\; of\; FeSO_4$ and $Fe_2(SO_4)_3$ having equal number of sulphate ion in both ferrous and ferric sulphate is :

Question

$\begin{array}{1 1} 1:2 \\ 3:2 \\ 2:3 \\ 2:1 \end{array}$