
Answer:
weight at height = 100 N .
Explanation:
The problem relates to variation of weight  due to change in height .
Let gâ and gâ be acceleration due to gravity , m is mass of the object .
At the surface :
Applying Newton's law of gravitation
mgâ = G Mm / R²
At height h from centre
mgâ = G Mm /h²
Given mgâ = 400 N
400 = G Mm / R²
400 = G Mm / (6400 x 10³ )²
G Mm = 400 x (6400 x 10³ )²
At height h from centre
mgâ =  400 x (6400 x 10Âł )²/ ( 2 x 6400 x 10Âł)²
= 400 / 4
= 100 N .
weight at height = 100 N