Chemistry question help?

The precipitates are supposedly Ca3(PO4)2 and Mg3(PO4)2. Your calcium and magnesium loads are 5.5x10-2 g-atoms and 9.5x10-2 g-atoms respectively, a total of 14.7x10-2 g-atoms. From the formulas, for each 3 g-atoms of these ion, 2 g-moles of PO4-3 are required. So you need AT LEAST 1x10-1 g-moles of phosphate, which comes as Na3(PO4). So you can compute the grams based on the sodium phosphate mol wt. From the Ksp equations, there must be PO4 in solution to depress the calcium and magnesium ion to the point you can consider it completely eliminated (say about 10-6 g-atoms total).