What is the concentration of the cation that precipitates first

you helped me with a problem yesterday. There is a second part to the same question and I am stuck.What is the concentration of the cation that precipitates first when the other cation just begins to precipitate.Consider a solution that is 2.2 x 10-2 M in Fe2 and 1.6 x 10-2 M in Mg2 .