114. Solving a chemistry problem

To build a calibration graph, V ml of a standard solution of manganese containing 1.25 mg/ml was placed in volumetric flasks and Mn was oxidized to MnO4-, photometrically and the following results were obtained:
V, ml 11.0 12.0 13.0 14.0 15.0
D 0.200 0.400 0.600 0.800 1.01

A sample of ore weighing 0.500 g was dissolved and the solution was diluted to 100.0 ml. Manganese was oxidized to permanganate in 50.0 ml of the filtrate and the solution was diluted to 250.0 ml. We measured the relative optical density as in the construction of a calibration graph. Calculate the mass fraction (%) of manganese in the ore sample if Dx = 0.320.