To 300gr of 20% sodium chloride solution, 40gr of the same salt was added. How has the mass

To 300gr of 20% sodium chloride solution, 40gr of the same salt was added. How has the mass fraction of a substance in solution changed?

300 g of 20% NaCl solution contains (20% * 300) / 100% = 60 g of salt.
In a new salt solution 60 + 40 = 100 g.
We calculate the concentration of the new solution if we divide the mass of salt by the total mass of the solution, multiplying by 100%:
100 / (300 + 40) * 100% = (100/340) * 100% = 29.41%.