When the salt was dissolved in 200 ml of water, a solution was obtained with a mass fraction of salt of 20%.

When the salt was dissolved in 200 ml of water, a solution was obtained with a mass fraction of salt of 20%. Another 150 ml of water was added to this solution. What is the mass fraction of salt in the resulting solution?

We have a 20% salt solution, with a mass of 200 ml, 150 ml of water was added to it, which means that the mass of our solution is now 350 ml. 200 ml of solution contains 20% salt, and 350 ml of solution X% salt, hence X = 150 * 20/200 = 15.
Answer: the mass fraction of salt in the resulting solution is 15%.