The density of gasoline is 0.7 g/cm3, and the density of water is 1 g/cm3. Thus the mass of the gasoline is 55*0.7 = 38.5g and the mass of the water is 60g.
Combining the 55 cm3 and 60 cm3 of substances with the aforementioned masses yields a volume of 55+60 = 115cm3 and a mass of 38.5+60 = 98.5g. The density is therefore 98.5/115 = 0.86 g/m3.
