Question

2NaI → Na₂SO₄

2 × 150 g 142 g

$\therefore xg \frac{142}{300} \times x\text{ g}$

2NaCl → Na₂SO₄

2 × 58.5 g 142 g

$\therefore(100 - x)g \frac{142}{117} \times (100 - x)g$

From the question, $\frac{142}{300} \times x + \frac{142}{117} \times (100 - x) = 100$

Hence, the percentage of NaI in the mixture = 28.86%