145. Solving a chemistry problem

63. In the determination of magnesium by the comparison method, a sample of the alloy weighing 0.2000 g was taken, dissolved in a mixture of acids and transferred to a 250.00 ml volumetric flask.
Before measuring the atomic absorption of magnesium, 5.00 ml of the sample was diluted to 200.0 ml. The atomic absorption of this solution at 285.2 nm was 42.0 scale units.
A standard solution of magnesium with a concentration of 1.5000 μg Mg in 1.00 ml has an atomic absorption of 55.0 scale divisions. Determine the mass fraction (%) of magnesium in the sample.