To 100 g of a solution with a mass fraction of magnesium nitrate of 15%, 5 g of the same salt was added and then 25 ml

To 100 g of a solution with a mass fraction of magnesium nitrate of 15%, 5 g of the same salt was added and then 25 ml of water was evaporated from it. Mass fraction of salt in the resulting solution is equal to

100 g of a 15% solution contains 100 * 0.15 = 15 g of salt
Therefore, its mass fraction in the final solution will be equal to:
(15 + 5) * 100 / (100 + 5-25) = 25%

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