Question

A bag contains a mixture of copper and lead BBs. The average density of the BBs is 9.80 {\rm g/cm^3}. Assuming that the copper and lead are pure, determine the relative amounts of each kind of BB.

Answer 1

You can do this more easily with a sheet of graph paper but this will have to do since we can't draw pictures.
11.34...............9.80.........8.93
d Pb...............d mixture.....d Cu
|....................|............|
100%Pb...............?..........0%Pb
0%Cu.................?........100% Cu

Look at the above as a graph.
The total span of density is from 11.34 to 8.93 = 2.41 units.
From Pb to mixture is 11.34-9.80 = 1.54
From mixture to Cu is 9.80-8.93 = 0.87
So % Cu = (1.54/2.41)*100 = ??
%Pb = (0.87-2.41)*100 = ??
Check my thinking.

Answer 2

I'm not sure you got the lead part right, whats simpler is once you get the copper do 1-copper answer to find the lead as the bb's are mixed together (must add up to 100%)