What is the mass fraction of salt in the resulting solution if 1 kg of water was added to 2 kg of a 15% salt solution?

As you know, the formula for finding the mass fraction of a substance in a solution:
w (substance) = m (substance) / m (solution),
to use this formula, we need to get rid of the percentages, w (salt) = 15% / 100% = 0.15
After adding water to the solution, we find the mass of the solution:
m1 (solution) = m (solution) + 1kg = 2 + 1 = 3 kg
And the formula for finding the mass fraction of a substance in a new solution:
w1 (substance) = m (substance) / m1 (solution)
The mass of salt has not changed for us, so we express it from both formulas and equate:
m (substance) = w (substance) * m (solution) = w1 (substance) * m1 (solution),
Let us express the required value from this expression:
w1 (substance) = w (substance) * m (solution) / m1 (solution),
Substitute the numbers:
w1 (substances) = 0.15 * 2/3 = 0.1
Or let’s move on to percentages 0.1 * 100% = 10%

Answer: the mass fraction in the new solution is 10%