I'm solving a problem: Two charges are given such that the distance between them is $r$ and intensity at point of other is $E$. If $q$ and $-3q$ are the charges, then the intensity at point of $-3q$ will be?
Please give some hints as I'm completely confused. I thought as there are two charges then the field of both the charges will act on unit positive test charge so the total intensity will be
where $E_1$ and $E_2$ are intensities due to $q$ and $-3q$ respectively and $K$ is the electrostatic constant. But distance between $q$ and test charge is $0$ and so $E_1$ becomes infinity.