What Determines the Repulsion Between Two Positively Charged Spheres?

[Ry122]

Each of two small non-conducting spheres is charged positively, the combined charge being 40C . When the two spheres are 50 cm apart, each sphere is repelled from the other by a force of magnitude 2.5 N. Determine the magnitude of the smaller of the two charges

my attempt:
use eq. F=kq1q2/d
2.5=((9x10^9)(40-q2)(q2))/.5
this gives me the wrong answer
what am i doing wrong?

 

[Pyrrhus]

You forgot r is square.