What mass of salt must be dissolved in 250 g of water to obtain a 10% solution?

Let’s determine how much salt needs to be added to the solution:
To begin with, we write down an expression to determine the mass fraction of a substance in a solution:
w (in-va) = (m (in-va) / m (solution)) * 100%
Let us express the mass of a substance from this expression:
m (in-va) = (w (in-va) * m (solution)) / 100%
m (solution) = m (in-va) + m (H2O)
We denote m (in-va) = x, then the mass of the solution:
m (solution) = x + m (H2O) = x + 250
Let’s substitute all this in the formula for determining the mass of a substance:
x = 10 * (x + 250) / 100
x = 0.1 * (x + 250)
x = 0.1 * x + 25
x-0.1x = 25
x * (1-0.1) = 25
x = 25 / 0.9 = 27.8 g
Answer: you need to take 27.8 g of salt.