A set of five solutions for a standard addition experiment are prepared by delivering 10 mL of unknown sample and increasing volumes of standard (0, 5, 10, 15, and 20 mL) into five separate 50-mL volumetric flasks. Analytical reagents are then added and each flask is diluted to volume. The solutions are then analyzed, and a calibration curve is obtained. The x-intercept of the curve is 3.90 mM. What is the concentration of the initial unknown sample?