When equal volumes of two metals are mixed together, the specific gravity of alloy is 4. When equal masses of the same two metals are mixed together the specific gravity of the alloy now becomes 3. The specific gravity of metals are 

In case of mixture, 
${\rho }_{mix}=\frac{m_1+m_2}{V_1+V_2}$ 
When equal volumes are mixed, 
 $4=\frac{V{\rho }_1+V{\rho }_2}{V+V}=\frac{{\rho }_1+{\rho }_2}{2}\mathrm{............}\left(1\right)$
When equal masses are mixed, 
$3=\frac{m+m}{\frac{m}{{\rho }_1}+\frac{m}{{\rho }_2}}=\frac{2{\rho }_1{\rho }_2}{{\rho }_1+{\rho }_2}\mathrm{..........}\left(2\right)$ 
$\therefore$  From (1) & (2) specific gravity of the metals are z and 6