Question

what at what height above the surface of the earth the weight of man with weight 20 kg if his weight on the surface of earth is 80 kg

Answer 1

G = 6.67 x 10^-11

g = GM÷R²

10 = Gx 80/R²

R² = 8G

Now

g = Gm/r²

10 =G x 20/r²

r = 2G

So if the man is kept at half the distance to that when he weighed 80 kg, the would weigh 20 kg.