A parallel plate capacitor with air between the plate has a capacitance of $15$ pF. The separation between the plates becomes twice and the space between them is filled with a medium of dielectric constant $3.5$. Then the capacitance becomes $x/4$ pF.
The value of $x$ is

Resistances are joint as shown in the figure. In the balanced condition, the current $I$ drawn from the battery is

In an experiment to find emf of a cell using potentiometer, the length of null point for a cell of emf $1.5$ V is found to be $60$ cm. If this cell is replaced by another cell of emf $E$, the length of null point increases by $40$ cm. The value of $E$ is $x/10$ V.
The value of $x$ is

A charged particle $q$ is accelerated by a potential difference of $V$ enters a region of uniform magnetic field of induction $B$ at right angles to the direction of the field. The charged particle completes semi circle of the radius $r$ inside the magnetic field. The mass of the charged particle is (all quantities are in SI units)