To 80 g of a solution with a mass fraction of salt of 10%, 40 ml of water was added to determine the mass fraction of salt.

The mass of the solution is 80 grams;

To find the mass of salt, it is necessary to multiply its mass fraction by the mass of the solution.

m salt = 80 x 0.1 = 8 grams;

Adding 40 grams of water to the solution (water density 1 gram / cm3), its mass will increase and will be 80 + 40 = 120 grams. The mass of salt in the new solution will not change.

The salt concentration in the new solution will be:

With salt 2 = 8/120 = 0.066 = 6.67%;