135. Solving a chemistry problem

10. To construct a calibration graph in the determination of manganese, 2.5 was selected; 5.0; 7.5; 10.0; 12.5 and 15.0 ml 0.0125 n. KMnO4 solution and diluted with water to 100 ml. The deviation on the scale of the galvanometer on a single-beam photocolorimeter for the background was 80 divisions, and for the indicated solutions 72.5; 64.5; 59.0; 62.5; 47.0 and 42.5 divisions. A 0.5 g sample of steel was dissolved, and after manganese oxidation, the solution was diluted with water to 50 ml. The deviation on the galvanometer scale for the resulting colored solution under the same conditions was 55.5 divisions. Determine the content of manganese in steel (%).