The weight of a body at the surface of earth is 18N . The weight of...

As height of the object is considerable compared to the radius of the earth, therefore approximation formula can not be used.

So weight at given height,

$F = m{g}^{'} = \dfrac{GMm}{{\left(R + \dfrac{R}{2}\right)}^{2}} = \dfrac{4}{9}\dfrac{GMm}{{R}^{2}}$

$⇒ m{g}^{'} = \dfrac{4}{9} ( mg )$

$= \dfrac{4}{9} \times 18$

$= 8N$