In making an alloy, a substance of specific gravity $s_1$  and mass $m_1$  is mixed with another substance of specific gravity $s_2$  and mass $m_2$ ; then the specific gravity of the alloy is

Specific gravity of alloy $=\frac{\text{ Density of alloy }}{\text{ Density of water }}$

$=\frac{\text{Mass of alloy }}{\text{Volume of alloy}\times \text{density of water }}$

$=\frac{{\mathrm{m}}_1+{\mathrm{m}}_2}{\left(\frac{{\mathrm{m}}_1}{{\mathrm{\rho }}_1}+\frac{{\mathrm{m}}_2}{{\mathrm{\rho }}_2}\right)\times {\mathrm{\rho }}_{\mathrm{w}}}=\frac{{\mathrm{m}}_1+{\mathrm{m}}_2}{\frac{{\mathrm{m}}_1}{{\mathrm{\rho }}_1/{\mathrm{\rho }}_{\mathrm{w}}}+\frac{{\mathrm{m}}_2}{{\mathrm{\rho }}_2/{\mathrm{\rho }}_{\mathrm{w}}}}=\frac{{\mathrm{m}}_1+{\mathrm{m}}_2}{\frac{{\mathrm{m}}_1}{{\mathrm{s}}_1}+\frac{{\mathrm{m}}_2}{{\mathrm{s}}_2}}$

$\left[\text{As specific gravity of substance }=\frac{\text{density of substance }}{\text{density of water}}\right]$