(a) $g^{'} = \frac{g}{\left( 1 + \frac{h}{R} \right)^{2}}$ $\frac{g}{4} = \frac{g}{\left( 1 + \frac{h}{R} \right)^{2}}$ $1 + \frac{h}{R} = 2 \Rightarrow \frac{h}{R} = 1$ $\Rightarrow h = R$ $\therefore h = 6400km$
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.