When two sphere will have maximum force then the charge will be equally divided among them
so here we can say
[tex]F = \frac{kq_1q_2}{r^2}[/tex]
here for maximum force
[tex]q_1 = q_2 = \frac{q}{2}[/tex]
now we have
[tex]F_{max} = \frac{kq^2}{4r^2}[/tex]
now we have to find the charge transferred so that force is 1/4 times the maximum force
[tex]\frac{1}{4}(\frac{kq^2}{4r^2}) = \frac{k(q_1)(q-q_1)}{r^2}[/tex]
here in above equation q1 is charge that is transferred
[tex]\frac{q^2}{16} = q q_1 - q_1^2[/tex]
[tex]q_1^2 - q q_1 + \frac{q^2}{16} = 0[/tex]
now by solving above equation we have
[tex]q_1 = \frac{q \pm \sqrt{q^2 - q^2/4}}{2}[/tex]
[tex]q_1 = 0.067q[/tex]
so we will have
[tex]\frac{q_1}{q} = 0.067[/tex]