100. Solving a chemistry problem

3. In the fluorimetric determination of riboflavin, a standard solution was prepared containing 0.4 μg of the substance in 1 ml of solution, and the fluorescence intensity of this solution was measured using a fluorimeter (Ilm (St) = 100). The analyzed solution was quantitatively transferred into a 100 ml volumetric flask, the volume of the solution was brought to the flask mark with distilled water, and the fluorescence intensity of the solution was measured under the same conditions: Ilm (X)=5. Calculate the mass of riboflavin in the analyzed solution.