physical chemistry - Can the solubility of a compound be used to estimate the degree of separation of a solute from a solution?

I'm confused exactly on how to use the concept of solubility to get the amount of separation from a certain solute in the solution.

This arises from attempting to solve a problem regarding this matter. The problem described is as follows:

$400$ grams of anhydrous sodium sulphate ($Na_{2}SO_{4}$) is dissolved in a liter of hot water. The solution is then let to cool carefully until reaching $20^{\circ}C$ to remain supersaturated with respect to the formed decahydrate $Na_{2}SO_{4}\cdot 10H_{2}O$. Then a small crystal of the latter salt is added to the solution, separating the excess of $Na_{2}SO_{4}$ dissolved, remaining a saturated solution. It is known that the saturated solution is equivalent to $19.4$ grams of $Na_{2}SO_{4}$ by $100\,mL$ of water. What amount of the decahydrate would had been separated?

The given alternatives are:

$\begin{array}{ll} 1.&249\,g\,\textrm{to}\,259\,g\\ 2.&318\,g\,\textrm{to}\,327\,g\\ 3.&689\,g\,\textrm{to}\,698\,g\\ 4.&721\,g\,\textrm{to}\,730\,g\\ 5.&890\,g\,\textrm{to}\,899\,g\\ \end{array}$

What I assumed is that:

$400\,g\,Na_{2}SO_{4}\times\frac{142+180\,g\,Na_2SO_{4}\cdot 10 H_{2}O}{142\,g\,Na_{2}SO_{4}}-100\,mL\frac{19.4\,g}{100\,mL}\approx 807\,g$

Therefore that would be the grams of sodium sulphate decahydrate but I'm not sure if that would be the ammount. I'm confused why do the alternatives features a range?. How does it appear that?. Can somebody explain how exactly to get to that given range?.