A small point of mass $m$ is placed at a distance 2 R from the centre ' $O$ ' of a big uniform solid sphere of mass $M$ and radius $R$. The gravitational force on ' $m$ ' due to $M$ is $F_1$. A spherical part of radius $R / 3$ is removed from the big sphere as shown in the figure and the gravitational force on m due to remaining part of $M$ is found to be $F_2$. The value of ratio $F_1: F_2$ is