Density questions? - maths?

Mass of liquid C
= (Mass of liquid A) + (Mass of liquid B)
= (140 g) + (128 g)
= 268 g

Volume of liquid C
= (Volume of liquid A) + (Volume of liquid B)
= [(Mass of liquid A) / (Density of liquid A)] + [(Mass of liquid B) / (Density of liquid B)]
= [(140 g) / (0.7 g/cm³)] + [(128 g) / (1.6 g/cm³)]
= (200 cm³) + (80 cm³)
= 280 cm³

Density of liquid C
= (Mass of liquid C) / (Volume of liquid C)
= (268 g) / (280 cm³)
= 0.957 g/cm³

Add masses to work out total; but you need to find volumes to get total volume

Volume of A = mass/density = 140 g/0.7 g/cm^3 = 200 cm^3

Volume of B = mass/density = 128 g/1.6 g/cm^3 = 80 cm^3

Density of Liquid C = total mass/total volume = 268/280 = 67/70 ~ 0.957

We're assuming there's no chemical reaction or change in temperature, pressure, etc. that would change the density of the substances.

140g of Liquid A has a volume of 140/0.7 = 200 cm^3
128g of Liquid B has a volume of 128/1.6 = 80 cm^3
Total mass of Liquid C: 140 + 128 = 268
Total volume of Liquid C: 200 + 80 = 280

Final answer: 268/280 = 67/70 =~ 0.95714285714285714285714285714286 g/cm^3