The discussion on this topic has died down.

After a week, it's time to explain what the error is. Calculating this problem is possible assuming the additivity of the volume. As Enthalpy rightly pointed out, there may be surprises, but for the total content of Zn + Sn = 5%, surprises are unlikely to be expected.

For both 5% of binary alloys of Cu with Sn or Zn, manufacturers give a density of 8.85 (assuming volume additivity, the calculated values are 8.860 and 8.846 respectively - i.e. the error is less than 2%).

There are no data for three-component alloys with such a low content of Zn and Sn, but with a high probability it can be expected that their density will be within these limits (8.846 to 8.860). And only for the density within such limits the problem is solvable. For Sn content 4 to 1% and Zn up to 5% I got densities 8.857, 8.855, 8.852 and 8.849, respectively.