a specimen of rich gold ore weighs 309.3 g and its volume is 82.9cc; quartz (g= 2.65) is the only waste mineral. The alloy contains 12.1% of silver, the density of the alloy being 17.5g/cc. how much pure gold are there in the specimen?

Sounds like a density problem, see if this helps:
Density of Gold (http://www.elmhurst.edu/~chm/vchembook/125Adensitygold.html)