Two identical charges, $5\mu C$ each, are in the air. What is the largest charge to be transferred from one charge to another, so that their interaction force decreases to 2.5 times?

Given
Charge  $q=5\text{\hspace{0.17em}μC}$

Let Q$\mu C$ is transferred from one charge to other.
We know coloumb force $F=\frac{k{q}^2}{r^2}\mathrm{.....}\left(1\right)$

$\Rightarrow \frac{k\times {\left(5\times {10}^{-6}\right)}^2}{r^2}=2.5\times \frac{k\left(5-Q\right)\left(5+Q\right)}{r^2}$

$\Rightarrow \frac{{\left(5\times {10}^{-6}\right)}^2}{2.5}=\left(5-Q\right)\left(5+Q\right)\times {10}^{-12}$

$\Rightarrow \frac{25\times {10}^{-12}}{2.5}=\left(25-Q^2\right)\times {10}^{-12}$

$\therefore Q=\sqrt{15}\text{\hspace{0.17em}μC}$