Lead in a sample was determined by Atomic Absorption using the method of standard addition. All the lead in a 25.00 mL portion of the sample was extracted into 10.00 mL of an organic solvent resulting in A = 0.315. To a second 25.00 mL part, 10.00 mg of lead was added and extracted into a second 10.00 mL portion of the same organic solvent. This solution gave A = 0.525. What was the concentration of lead, in mg/mL, in the original sample?