141. Solving a chemistry problem

A sample of fertilizer weighing 2,000 g was treated by boiling with a saturated solution of ammonium oxalate; after cooling, the solution was diluted in a volumetric flask with a capacity of 500.0 ml and filtered. A 5.00 ml aliquot of the filtrate was diluted to 250.0 ml. The resulting solution was photometric in flame, as well as standard solutions prepared from KCl. Photometry data are given in the table.
Option number 1 (for the first sample)
W (K)% 5.00 10.00 15.00? ? ?
Irel 8.0 15.0 24.2 12.7, 18.4, 20.8
Build a calibration graph and determine the mass fraction (%) of potassium in the fertilizer.