At what height above the earths surface would the acceleration due to gravity be half of what it is on the surface take radius of Earth to be R?

A stone is allowed to fall from the top of a tower 100 m high and at the same time another stone is projected vertically upwards from the ground with a velocity of 25 m/s. Find when and where the two stones meet?

Suppose the two stones meet at a height x from the ground, after time t from the start.

Hence, the two stones meet after 4 s at a height of 20 m from the ground or 80 m from the top.