Physcis electrostatic force problem?

Question

A charge of +q is located at the origin, while an identical charge is located on the x axis at x = 0.45 m. A third charge of +5 q is located on the x axis at such a place that the net electrostatic force on the charge at the origin doubles, its direction remaining unchanged. Where should the third charge be located?
x = ? m

Answer 1

Charge repulsion falls off as the square of the distance, so for a +5q charge to offer the same repulsion as a +1q charge, (thus doubling the force), it will have to located further out.
1q/0.45m = 5q/(x^2)
x^2 = 5*0.45
x = sqrt( 2.25 ) = 1.5
the 5q charge will have to be located at +1.5 meters on x axis.