Answer
When we equate the electrostatic force and the gravitational force, the $r^2$ cancels out of the equation.
Work Step by Step
The electrostatic force is proportional to $\frac{1}{r^2}$, where $r$ is the distance separating the charges.
The gravitational force is proportional to $\frac{1}{r^2}$, where $r$ is the distance separating the objects.
When we equate the electrostatic force and the gravitational force, the $r^2$ cancels out of the equation.
The required positive charge does not depend on the separation distance.
