40 ml of water and 21 g of the same salt were added to 90 g of a solution with a mass fraction of 10% magnesium chloride

40 ml of water and 21 g of the same salt were added to 90 g of a solution with a mass fraction of 10% magnesium chloride. Mass fraction of salt in the resulting solution is equal to

ω = m (substance) / m (solution)
Find the mass of the substance m = 90 * ω + 21 = 30 g
Find the mass of the solution m = m (initial) + m (added) = 90 + 40 + 21 = 151 g
ω (final) = 30 * 100/151 = 20%

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