The quantity of pure gold in 50g of an alloy of gold and copper is, if the densities of gold, copper and alloy are $\text{19}.0{\text{gcm}}^{-\text{3}},\text{9}.0{\text{gcm}}^{-\text{3} }$ and $16{\text{gcm}}^{-\text{3}}$ respectively   

Given that an alloy contains Gold & Copper.

${\rho }_A=16\text{g/cc}\text{,}{\rho }_g=19\text{g/cc}\text{,}{\rho }_C=9\text{g/cc}$

Asked to find amount of pure gold in

Mass of alloy = 50 g

Mass of gold = m

Mass of copper $=50-\text{m}$

We can have

Volume of alloy = Volume of gold + Volume of copper

${\text{V}}_{\text{A}}={\text{V}}_{\text{G}}{\text{+V}}_{\text{C}}$

$\frac{50}{16}=\frac{m}{19}+\frac{50-m}{9}$

$\frac{50}{16}=\frac{950-10m}{171}$

$\text{855}0=\text{152}00-\text{16}0\text{m}$

$\text{m}=\text{41}.\text{6g}$  $=3230\text{N}$