[vanderwaals]

How many grams of solid NaCl is needed to increase the concentration of a 400.0 g NaCl
solution from 18.0% (w/w) to 20.0% (w/w)?

The answer key says 10.0 g, but I calculated 55.6 g and I can't figure out what I'm doing wrong.
First, I solved (400/x) = 0.18 to get the total mass of the solution, which was 2222.22. I then set y equal to the mass of NaCl added in the equation (400+y)/(2222.22+y) = 0.20. I got y to be 55.6. What am I doing wrong?

[vanderwaals]

Never mind, I got it. I misinterpreted "400 g NaCl solution" as 400 g of NaCl in the solution and not the total mass.