54. Solving a chemistry problem

A sample of brass weighing 0.3280 g was dissolved in HNO3, the solution was diluted to 500.0 cm3.
1) 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.
2) A second aliquot of 25.0 cm3 was masked with Cu2+; 27.6 cm3 of EDTA was used for titration of Pb2+ and Zn2+.
3) The third 100.0 cm3 aliquot was masked with Cu2+ and Zn2+; 10.8 cm3 of EDTA was used for Pb2+ titration. Determine the content (%) of Cu, Zn and Pb in brass.