A neutron star has a mass of 2.0 x 10 ky and a diameter of 20 km. (a) What is the the mean density of the star? (b) Calculate the gravitational field strength at the surface. (c) The mass and diameter of a planet in orbit around the neutron star are both 4 times those of the Earth. Calculate the ratio of the acceleration of free fall on the planet's surface to that on the Earth's surface (you should assume that each body is a sphere of uniform density)