For what it’s worth, with three variables, especially knowing that antimony never gets whst? above 6%? You could actually do the calculation using antimony as a fixed 4%, and could very easily come within one or 2% on the actual alloy if you knew that the alloy was only lead and tin.
Certain variables dominate the equation.In other words, in terms of their overall effect within a weighted average, they behave as though they are fixed. Or at least contribute so little, that you can treat them as though they are fixed.
