90. For the luminescent determination of riboflavin (vitamin B2) in a food product by the addition method, 0.1500 g of the product was dissolved and, after appropriate treatment, the luminescence intensity of the resulting solution was measured. It turned out to be 25. After adding a standard solution containing 20 µg of vitamin B2, the luminescence intensity increased to 60. Determine the mass fraction (ω,%) of vitamin B2 in the product if the luminescence intensity of the blank solution is 12.

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