Question

Is material that has a density of 0.2g/ml good for building a bridge or for making couch cushions

Answer 1

D = 0.2 g / ml = 0.2 g / cm³
For example, density of steel is 7.85 g / cm³.
Density of pure water is 1.0 g/cm³. An object which has a density < 1.0 g/cm³ will float in water.
Answer: Material that has a density of 0.2 g/ml ( 0.2 g/cm³ ) is good for making couch cushions. 

Answer 2

The density of an object refers to its mass per unit volume. The density of an object is determined by its mass, its size and arrangement of its atom. The mathematical formula for density is given by: Density = Mass / volume. The density of a substance is part of the determinant of what the substance can be used for. The density of an object depends on how tightly its molecules are packed. A substance with loosely packed molecules will have low density while a substance with tightly packed particles will have high density.

A material with 0.2g/ml density have low density, thus, such an object will not be suitable for holding large weight. Therefore it will be suitable for making couch cushion.