Mn was used as an internal standard for measuring Fe by atomic absorption spectroscopy. A standard mixture containing 2.00 ppm and 2.50 ppm gave a Fe / Mn signal ratio (quotient) of 1.05. A 5.00 mL unknown Fe solution was mixed with 1.00 mL solution containing 12.5 ppm Mn. The absorbance of this mixture at the Mn wavelength was 0.128 and the absorbance at the Fe wavelength was 0.128. Find the concentration of the unknown Fe solution.