An ion-selective electrode and a reference electrode were placed in exactly 100.0 mL of the sample; a reading of 21.6 mV between the two electrodes was obtained. After the addition of exactly 10.0 mL. of a standard solution with a concentration of 100 ppm, the ion selective electrode gave a reading of 45.7 mV with respect to the reference electrode. The response slope of the indicator electrode was previously determined to be 58.8 mV. What is the sample concentration?