Explanation

(b) Weight of body on the surface of earth $mg = 12.6\ N$
At height $h,\ $the value of $\ g'$ is given by
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ g^{'} = g\frac{R^{2}}{{(R + h)}^{2}}$
$Now,\ \ \ \ \ \ \ \ \ \ \ h = \frac{R}{2}$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ g^{'} = g\left( \frac{R}{R + (R/2)} \right)^{2} = g\frac{4}{9}$
$Weight\ at\ height\ h = mg\frac{4}{9}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 12.6 \times \frac{4}{9} = 5.6\ N$