When air is replaced by a dielectric medium of constant $k$, the maximum force of attraction between two charges separated by a distance

Two point charges $ + 3\,\mu C$ and $ + 8\,\mu C$ repel each other with a force of $40\,N$. If a charge of $ - 5\,\mu C$ is added to each of them, then the force between them will become....$N$

A point charge $q_1$ exerts force $F$ upon another point charge $q_2$. If a third charge $q_3$ be placed near the charge $q_2$, then the force that charge $q_1$ exerts on the charge $q_2$ will be

Two free point charges $+q$ and $+4q$ are a distance $R$ apart. $A$ third charge is so  placed that the entire system is in equilibrium. Then the third charge is :-

Two positive charges of $20$ $coulomb$ and $Q\;coulomb$ are situated at a distance of $60\,cm$. The neutral point between them is at a distance of $20\,cm$ from the $20\,coulomb$ charge. Charge $Q$ is.....$C$

Two point charges $ + 9e$ and $ + e$ are at $16\, cm$ away from each other. Where should another charge $q$ be placed between them so that the system remains in equilibrium