Question

a person weighs 80N on the surface of the Earth . What should be the distance of the person from the centre of the Earth , such that his weight is 5N ? Radius of the earth is 6400km.​

Answer 1

The other answers are correct and well written. Another approach would be to solve the problem "from scratch " using the definition of gravity

An object that weighs 80N on the surface of the earth would have a mass of 80/9.82 = 8.147 kg.

How far out it would have to go to weigh 20N is given by

sqrt( (6.67e-11 x 5.98e24 x 8.147)/20) =

1.27e7 meters from the center of the earth.

Subtract the earth's radius 6.371e6 and you get 6.375e6 meters, which is one earth radius.

Back to basics. This works for any object for which you know the mass.