For an open circuit condition,what is the value of the terminal voltage of a cell?

Thousand cells of same emf $E$ and same internal resistance $r$ are connected in series in the same order without an external resistance. The potential drop across $399$ cells is found to be ......... $E$.

$A$ battery of emf $E$ and internal resistance $r$ is connected with an external resistance $R$ as shown in the figure. The battery will act as a constant voltage source if

$A$ student is given $4$ identical batteries having $EMF$ $1.5 \ V$ each and internal resistance of $0.1 \ \Omega$ each. The student is asked to connect them in an assisting manner. By mistake,he connects $1$ battery in reverse way. The resultant $EMF$ and resultant internal resistance offered by the combination is . . . . . . .

Two batteries,one of emf $18 \, V$ and internal resistance $2 \, \Omega$ and the other of emf $12 \, V$ and internal resistance $1 \, \Omega$,are connected in parallel as shown. The voltmeter $V$ will record a reading of ............. $V$.

As shown in the figure,a group of $N$ cells is connected in series,where the $emf$ of each cell $E_n$ varies with its internal resistance $r_n$ according to the relation $E_n = 1.5 r_n$. The current $I$ in the circuit is ................ $A$.