What is the current in $A$ flowing through the given circuit?

In the given circuit,the reading of the ideal voltmeter is $\frac{E}{2}$. Find the internal resistance $r$ of the cell in $\Omega$.

$A$ $dc$ source of $emf \, E_1 = 100 \, V$ and internal resistance $r = 0.5 \, \Omega$,a storage battery of $emf \, E_2 = 90 \, V$,and an external resistance $R$ are connected as shown in the figure. For what value of $R$ will no current pass through the battery $E_2$ (in $.5$)? ................ $\Omega$

In the given figure, the current through the $4 \, \Omega$ resistor is ................. $A$.

$A$ cell of constant $e.m.f.$ is connected first to an external resistance $R_1$ and then to an external resistance $R_2$. If the power consumed in both cases is the same,what is the internal resistance of the cell?

$A$ voltmeter of resistance $150 \Omega$ connected across a cell of e.m.f. $3 \ V$ reads $2.5 \ V$. What is the internal resistance of the cell (in $\Omega$)?