In a post office box arrangement used to determine the value of an unknown resistance,the unknown resistance should be connected between:

In the given part of a circuit,the potential at point $B$ is zero. Then the potentials at $A$ and $C$ respectively are

Calculate the net resistance of the circuit between $A$ and $B$.

The figure shows a part of an electric circuit. The potentials at points $a$,$b$,and $c$ are $30\,V$,$12\,V$,and $2\,V$ respectively. The current through the $20\,\Omega$ resistor will be $........\,A$.

The value of the resistance $R$ in the figure is .......... $\Omega$.

Write the necessary facts to understand Kirchhoff's laws.