As shown in the figure , the voltmeter reads 2 V across 5$\mathrm{\Omega }$ resistor. The resistance of the voltmeter is _______ $\mathrm{\Omega }$

$\begin{array}{l} \\ \mathrm{Potential} \mathrm{across} 2\mathrm{\Omega } \mathrm{is} 1\mathrm{V}\\ \mathrm{I}=\frac{\mathrm{V}}{\mathrm{R}}=\frac{1}{2}\mathrm{A}\\ \\ \mathrm{Current} \mathrm{through} 2\mathrm{\Omega } \mathrm{is} {\mathrm{I}}_1=\frac{2}{5}\mathrm{A}\end{array}$

$\begin{array}{l}\mathrm{Current} \mathrm{through} \mathrm{voltmeter}\\ {\mathrm{I}}_2=\mathrm{I}-{\mathrm{I}}_1=\frac{1}{2}-\frac{2}{5}\\ {\mathrm{I}}_2=\frac{1}{10}\mathrm{A}\\ \Rightarrow {\mathrm{R}}_{\mathrm{v}}=\frac{\mathrm{v}}{{\mathrm{I}}_2}\\ {\mathrm{R}}_{\mathrm{v}}=\frac{2}{\left(\frac{1}{10}\right)}=20\mathrm{\Omega }\end{array}$