40% (wt./vol.) NaCl has specific gravity 1.12 g/mL. In ppm the concentration of NaCl is

100 mL solution = 100 × 1.12 g solution = 112 g solution

That is, 72 g H₂O has 40 g NaCl

$\therefore \frac{40}{72}\times {10}^6=5.5\times {10}^5\mathrm{ppm}$