Find the mass of salt that must be dissolved in 200 g of water to obtain a 10% solution.

The mass of the solution m (p-pa) is the sum of the mass of the solvent m (p-la) (in our case, water) and the mass of the solute m (p.v.) (In our case, salt), m (p-pa) = m (p.v.) + m (r-la). The ratio of the mass of the solute to the mass of the solution is called the mass fraction of the solute – w = m (r.v) / m (r-ra).
We transform the formula taking into account the expression for m (solution):
w = m (r.v.) / (m (r.v.) + m (r-la)), and we express m (r.v.) from here.
w * m (p.v.) + w * m (r-la) = m (p.v.)
m (p.v.) * (1-w) = w * m (r-la)
m (p.v.) = w * m (r-la) / (1-w)
m (p.v.) = 0.1 * 200 / (1 – 0.1) = 22.22 g