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 + m2 )/(s1 +s2 ) (b) ( s1s2 ) / ( m1 + m2 )
(c) ( m1 + m2 ) / [(m1 / s1 ) + ( m2 / s2 )] (d) [(m1 / s1 ) + ( m2 / s2 )] / ( m1 + m2 )
Answer-c
(5)