141. Solving a chemistry problem

From a 0.2542 g sample of steel, after appropriate processing, 100.0 ml of a colored solution of oxidized nickel dimethylglyoximine was obtained. The relative optical density of this solution A / x turned out to be 0.55. To construct a calibration graph, we took three standard solutions with a nickel content of 8.0; 10.0 and 12.0 mg in 100.0 ml, photometrically measured them and obtained relative optical densities of 0.24, respectively; 0.46 and 0.70. The reference solution contained 6.0 mg nickel in 100.0 ml. Determine the percentage of nickel in steel.