From a solution obtained by dissolving 30.4 g of iron sulfate (2) in 349.6 g of water, it is necessary to evaporate 

From a solution obtained by dissolving 30.4 g of iron sulfate (2) in 349.6 g of water, it is necessary to evaporate  _ g of water in order to increase the concentration of iron sulfate by 25%

Let’s write down given:
m (FeSO4) = 30.4 g
m (H2O) = 349.6 g
w (FeSO4) = 25% or 0.25
m (H2O) -?
Decision :
1) Calculate the mass of the FeSO4 solution;
m solution (FeSO4) = m (FeSO4) + m (H2O)
m solution (FeSO4) = 30.4 g + 349.6 g = 380 g
2) Mass fraction of a substance in a solution:
w = m (substance) * 100% / m solution;
3) Let the mass of the evaporated water be x g, then the mass of the new solution will be equal to:
m solution (FeSO4) = 380 g – x g (Н2О), and
w (FeSO4) = (m (FeSO4) * 100%) / m solution (FeSO4);
0.25 = 30.4 / (380 – x)
95 – 0.25x = 30.4
0.25 x = 64.6
x = 258.4 g (H2O)
Answer: 258.4 g (H2O)