4. In making an alloy, a substance of specific gravity and mass is mixed with another substance of specific gravity and mass ; then the specific gravity of the alloy is

s₁ and mass m₁ is mixed with another substance of specific gravity s₂ and mass m₂ ; then the specific gravity of the alloy is

(a)   ( m₁ + m_{2 )}/(s₁ +s₂ )                                                          (b)  ( s₁s₂ ) / ( m₁ + m_{2 )}

(c)  ( m₁ + m_{2 )} / [(m1 _/ s₁ ) + (  m₂  /  s₂ )]                           (d)  [(m1 _/ s₁ ) + (  m₂  /  s₂ )]  /  ( m₁ + m_{2 )}

Answer-c