Liquid A has a density of 0.7 g/cm^3 Liquid B has a density of 1.6 g/cm&3 140g of liquid A and 128g of liquid B are mixed to make liquid C?

Method 1 :

By definition: Density = Mass / Volume
Then: Volume = Mass / Density

Mass of liquid A = 140 g
Volume of liquid A = (140 g) / (0.7 g/ cm³) = 200 cm³

Mass of liquid B = 128 g
Volume of liquid B = (128 g) / (1.6 g/cm³) = 80 cm³

Mass of liquid C = (140 + 128) g = 268 g
Volume of liquid C = (200 + 80) cm³ = 280 cm³
Density of liquid C = (268 g) / (280 cm³) = 0.957 g/cm³

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Method 2 :

Density of liquid C = Mass / Volume = (140 + 128) / [(140/0.7) + (128/1.6)] g/cm³ = 0.957 g/cm³

Assuming volumes are conserved in mixing then the strategy is as follows

remember d = m/V

first use the equation to find the volume of A and B from their masses and densities

next calculate the total mass and total volume

finally use the equation to find the density of C because you now know the total mass and total volume of C

You actually can't answer this. Some liquids interact with each other in such a way that volume isn't conserved. So when combining the two liquids the volume of the resulting liquid isn't always the volumes of the original liquids added together.