A particle is taken to a height R above the earths surface where R is the radius of the earth. The acceleration due to gravity there is

h=R

Acceleration due to gravity on the surface of earth $=\frac{GM}{R^2}$

                                                                                              $=9.8 m{s}^2$

Acceleration due to gravity at a height (h) from the surface of earth $=\frac{GM}{{\left(R+h\right)}^2}$

            $\frac{GM}{{\left(R+R\right)}^2}==\frac{GM}{{\left(2R\right)}^2}=\frac{GM}{4R^2}=\frac{9.8}{4}=2.45m{s}^{-2}$