96. Solving a chemistry problem

In standard solutions of potassium salt with a concentration of SA, the electrode potentials of the potassium selective electrode relative to the silver chloride electrode were measured and the following data were obtained:
Sk, mol/l 1.0*10-1 1.0*10-2 1.0*10-3 1.0*10-4
E, mV 100 46.0 -7.00 -60.0
Based on these data, a calibration graph was built in the coordinates E - pCK. A weighed sample weighing 0.2000 g containing potassium was dissolved in water and the volume was adjusted to V(ml). Then we measured the electrode potential of the potassium selective electrode (EX) in the resulting solution:
V, ml 500
EX, mV -57
Calculate the mass fraction (%) of potassium in the sample.