Let ρ1 and ρ2 be the densities of two liquids.
When equal volumes of two liquids are mixed, then special gravity (ρ) is given by
ρ=(ρ1+ρ2)/2
Here, ρ1 and ρ2 are the densities of two liquid.
7/2=(ρ1+ρ2)/2
⇒ρ1+ρ2=7 .....(1)
When equal masses of the same two liquids are mixed, then special gravity (ρ) is given by
ρ=2ρ1ρ2/(ρ1+ρ2)
⇒12/7=2ρ1ρ2/(ρ1+ρ2)
⇒6ρ1+6ρ2=7ρ1ρ2 .....(2)
Solving equation (1) and (2), we have
ρ1=6 kg m^−3 and
ρ2= 1 kg m^−3