In the circuit shown in the figure, no current flows through the ideal ammeter. If the internal resistance of the cell is negligible, the value of unknown resistance $R $ is .............. $\Omega$
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If no current flows through the ammeter $(1),$ potential on the $20 \mathrm{\,W}$ resistance becomes $2 \mathrm{\,V}.$
$\Rightarrow \mathrm{R}=\frac{4 \mathrm{V}}{0.4 \mathrm{\,A}}=10\, \Omega$
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