Charge of $2Q$ and $-Q$ are placed on two plates of a parallel plate capacitor if capacitance of capacitor is $C$ find potential difference between the plates

The plates of a parallel plate capacitor are charged up to $100\,volt$. A $2\,mm$ thick plate is inserted between the plates, then to maintain the same potential difference, the distance between the capacitor plates is increased by $1.6\,mm$. The dielectric constant of the plate is 

The equivalent capacitance of the system of capacitors between $A$ and $B$ as shown in the figure

A thin spherical conducting shell of radius $R$ has a charge $q.$ Another charge $Q$ is placed at the centre of the shell. The electrostatic potential at a point $p$ a distance $R/2$ from the centre of the shell is

Consider a cube of uniform charge density $\rho$. The ratio of electrostatic potential at the centre of the cube to that at one of the corners of the cube is

In the figure a capacitor is filled with dielectric. The resultant capacitance is