Using density to find the % composition of an alloy

[Kushal]

Homework Statement

The densities (at 25*C) of tin and lead are 7.3 g/cm3 and 11.3 g/cm3 respectively. Plumber's solder is an alloy of tin and lead and has a density of 9.7 g/cm3.

Estimate the composition by mass, of the two metals in this solder.

Homework Equations

The Attempt at a Solution

i assumed the % composition of lead to be X%. so % composition of tin is (100 - X)%.
using [(X*11.3)/100] + [((100-X)*7.3)/100] = 9.7

i got % lead as 60% and % tin as 40%.

but the answers from the book are 30.1% for tin and 69.9% for lead.

any help!

 

[Mapes]

Amount fractions can be measured in weight percent (wt%) or atomic percent (at%), so just writing "%" is ambiguous. Perhaps this explains the difference.