The element Yttrium (Y) is often used as an internal standard in atomic emission (AES) measurements to account for sample-to-sample variability. A standard mixture containing 10.00 ppb Y and 20.00 ppb of Pb gave signals of 12,163 (Y) and 15,459 (Pb). A mixture with a volume of 10.00 mL was prepared by mixing 5.00 mL of unknown water with 5.00 mL of 25.00 ppb Y. This sample gave signals of 10,563 (Y) and 6177 (Pb). What is the concentration of Pb in the unknown water