A mixture of NaNO3 and Na2SO4 weighs 5.37g and contains 1.61g of sodium. What is the percentage of NaNO3 in the mixture?

Let y g be the mass of NaNO₃ in the mixture.
Then, mass of Na₂SO₄ in the mixture = (5.37 - y) g

Mass fraction of Na in NaNO₃ = 23.0/(23.0 + 14.0 + 16.0×3) = 23.0/85.0
Mass fraction of Na in Na₂SO₄ = 23.0×2/(23.0×2 + 32.1 + 16.0×4) = 46.0/142.1

Mass of Na:
y(23.0/85.0) + (5.37 - y)(46.0/142.1) = 1.61
0.270588y + 1.73835 - 0.323716y = 1.61
0.053128y = 0.12835
y = 2.42

Percentage of NaNO₃ = (2.42/5.37) × 100% = 45.1%