<s>[INST] There are two charges $Q_A=30\mathrm{pC}$ and $Q_B=40\mathrm{pC}$ separated by $r=2\mathrm{mm}$. The charges $Q_A$ and $Q_B$ are separated by two insulating sheets that have a very high resistance to electric current. An additional charge of $15\mathrm{pC}$ is added to $Q_A$ so that it becomes $Q_A'=45\mathrm{pC}$. Assume $Q_B$ stays unchanged.[/INST]