According to Coulomb's Law,which is the correct relation for the following diagram?

Two point charges $3 \times 10^{-6} \, C$ and $8 \times 10^{-6} \, C$ repel each other by a force of $6 \times 10^{-3} \, N$. If each of them is given an additional charge $-6 \times 10^{-6} \, C$,the force between them will be

Two insulated charged metallic spheres $P$ and $Q$ of negligible radii have their centres separated by a distance of $60 \,cm$. The mutual force of electrostatic repulsion, if the charge on each is $6 \times 10^{-7} \,C$, is

Charges $4Q$,$q$,and $Q$ are placed along the $x$-axis at positions $x = 0$,$x = l/2$,and $x = l$,respectively. Find the value of $q$ such that the net force on charge $Q$ is zero.

Two point charges $Q$ each are placed at a distance $d$ apart. $A$ third point charge $q$ is placed at a distance $x$ from the mid-point on the perpendicular bisector. The value of $x$ at which charge $q$ will experience the maximum Coulomb force is ...............

Two charged spheres are suspended by strings of equal length. The strings make an angle of $30^{\circ}$ with each other. When suspended in a liquid of density $0.8 \ g/cm^3$,the angle remains the same. The dielectric constant of the liquid is ........ (The density of the material of the sphere is $1.6 \ g/cm^3$.)