The final molarity of the Na⁺ ion is 0.102 mol/L.
NaCl → Na⁺ + Cl⁻
To get the molarity, you divide the moles of Na⁺ by the litres of solution.
Moles of Na⁺ =
0.020 36 mol Na⁺ (3 significant figures + 1 guard digit).
The ammonium sulfate contains no Na⁺ ions, and it does not form a precipitate with NaCl. Its only function is to dissolve the NaCl.
I assume that there is no volume change on adding the NaCl to the solution. Then