15. Solving a chemistry problem

149. A sample of brass weighing 0.3280 g was dissolved in HNO3, the solution was diluted to 500.0 cm3. For the titration of Pb2+, Zn2+ and Cu2+ in 10.0 cm3 of solution, 37.5 cm3 of 0.0025 mol/dm3 of EDTA solution was used. Cu2+ was masked in another portion of 25.0 cm3; 27.6 cm3 of EDTA was used for titration of Pb2+ and Zn2+. In the third portion, 100.0 cm3 in volume, Cu2+ and Zn2+ were masked; 10.8 cm3 of EDTA was used for Pb2+ titration. Determine the content (%) of copper, zinc and lead in brass.