What is the average density of a neutron star that has the same mass as the sun but a radius of only 20.0 km?

Explanation:

Density is a characteristic property bodies and materials, and is defined as the relationship between the mass and the volume , as shown below:

 (1)

In the case of the neutron star, we know it has the same mass as the Sun:

And its radius is

On the other hand, if we assume this neutron star has a spherical shape, its volume  can be calculated by the following formula:

  (2)

  (3)

  (4)

Substituting (4) in (1):

 (5)

Finally: