Two salts A _ 2 X and MX have the same value of solubility product...

For ${A}_{2}X$

$\begin{cases}{A}_{2}X & \to & 2{A}^{+} & + & {X}^{2 -} \\ 2{S}_{1} & {S}_{1}\end{cases}$${K}_{sp} = 4{S}_{1}^{3} = 4 \times {10}^{- 12}$

${S}_{1} = {10}^{- 4}$

for $MX$

$\begin{cases}MX & \to & {M}^{+} & + & {X}^{-} \\ {S}_{2} & {S}_{2}\end{cases}$

${K}_{sp} = {S}_{2}^{2} = 4 \times {10}^{- 12}$

${S}_{2} = 2 \times {10}^{- 6}$

So $\dfrac{{S}_{{A}_{2}x}}{{S}_{MX}} = \dfrac{{10}^{- 4}}{2 \times {10}^{- 6}} = 50$