The density of copper is ##8.96 "g/cm"^3"##. The density of zinc is ##7.14 "g/cm"^3"##. A sample of brass has 70% copper and 30% zinc by mass. What would be the density of this brass sample?

The of the alloy should simply be the weighted average of the individual densities.

So the weighted average:

##8.96## ##g## ##cm^-3## ##xx## ##0.7## ##+## ##7.14## ##g## ##cm^-3## ##xx## ##0.3## ##=## ##??##

Note that the property of brass that is prized (apart from the fact that you make horns and tubas out of it) is that the alloy is non-sparking. Given the fact that it doesn't give rise to sparks, it can safely be used in gas fittings (where a spark might be catastrophic!).