In the circuit shown the readings of the ammeter and voltmeter are $4 \mathrm{A}$and $20 \mathrm{V}$respectively. The meters are non-ideal, then $R$ is

In the circuit shown in figure reading of voltmeter is $V_{1 }$when only $S_1$ is closed, reading of voltmeter is $V_2$ when only $S_2$ is closed and reading of voltmeter is $V_3$when both $S_1$ and $S_2$ are closed. Then 

If the reading of ammeter ${\mathrm{A}}_3$ in figure is $0.75 \mathrm{A}$. Neglecting the resistances of the ammeters, the reading of ammeter ${\mathrm{A}}_2$ will be

If the reading of ammeter ${\mathrm{A}}_3$ in figure is $0.75 \mathrm{A}$. Neglecting the resistances of the ammeters, the reading of ammeter ${\mathrm{A}}_1$ will be

The resistance of the rheostat shown in figure is $30 \mathrm{\Omega }.$ Neglecting the ammeter resistance, the ratio of minimum and maximum currents through the ammeter, as the rheostat is varied, will be 

Between which points should the terminals of unknown resistance be connected in a post office box arrangement to get its value?