The figure below represents the two identical spheres.

The force between the charged sphere is,

$F=\frac{kqq}{r^2}$

If $10\%$ of electrons are transferred from one sphere to
another, then the charge become on $I$sphere is
$\left(q+\frac{q}{10}\right)=\frac{11}{10}q$

And the charge on $II$ sphere is $\left(q-\frac{q}{10}\right)=\frac{9}{10}q$

The force between the spheres is,

$F^{\text{'}}=\frac{k\left(\frac{11q}{10}\right)\left(\frac{9q}{10}\right)}{r^2}$

$F^{\text{'}}=\frac{99}{100}\frac{kqq}{r^2}$

$F^{\text{'}}=\frac{99}{100}F=0.99F$