Answer:
Because photosynthesis is a source of oxygen, it seems intuitive that photosynthesis evolved first: once enough O2 was in the air, then respiration would be able to arise in the newly oxygen-enriched atmosphere. However, some biologists have argued since the 1970's that respiration in fact evolved first.
To find the density of an object, divide the mass by the volume. To find the density of the object in your problem, use a calculator to divide 60.0 by 400.0