83. Solving a chemistry problem

2. A standard solution of cerium was prepared by dissolving 0.0500 g in 250.0 ml of a mixture of perchloric and hydrochloric acids. In flasks with a capacity of 25.00 ml were placed: 2.50; 3.00; 3.50; 4.00; 4.50; 5.00 ml of standard solution, added ammonia, hydrogen peroxide, nitrilotriacetic acid and diluted to the mark with water. When photometry relative to the first solution received:
Vst, ml 3.00 3.50 4.00 4.50 5.00
Aotn 0.135 0.270 0.405 0.545 0.680
A portion of lanthanide oxides weighing 0.0500 g was dissolved, the necessary reagents were added, diluted to 25.00 ml, and photometrically measured, as in the construction of a calibration graph.
Calculate the mass fraction of lanthanide oxides in the sample from the following data.
Arel.x = 0.440