What is the value of acceleration due to gravity at a height equal to half the radius of the Earth,from its surface?

If a particle takes $t$ seconds less and acquires a velocity of $v \text{ m/s}$ more in falling through the same distance on two planets,where the accelerations due to gravity are $2g$ and $8g$ respectively,then:

The acceleration due to gravity near the surface of a planet of radius $R$ and density $d$ is proportional to

As we move from the poles of the Earth towards the equator,what happens to the value of the acceleration due to gravity $(g)$?

The angular speed of the Earth,such that an object on the equator may appear weightless,is ($g = 10\,m/s^2$,radius of the Earth $R = 6400\,km$).

Assuming Earth to be a sphere of uniform density,what is the value of gravitational acceleration in a mine $100 \, km$ below the Earth's surface? (Given $R = 6400 \, km$,$g = 9.8 \, m/s^2$)