When determining lead in hydrochloric acid, 0.2 ml of the analyzed solution with a density of 1.19 g/ml was frozen with liquid nitrogen and the optical density Ax=20 was taken when the solution was defrosted. Similarly, the optical density of the analyzed solution was taken with the addition of 0.40 ml of a lead solution containing 0.02 μg Pb/ml (Ax=40). Calculate the mass fraction of lead (%) in the analyzed sample.

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