The reading of an ideal voltmeter in the circuit shown is,
 

$\begin{array}{l}\text{ Current in }\mathrm{APB}{=}_{50}^2\mathrm{A}\\ \text{ Current in }\mathrm{AQB}{=}_{50}^2\mathrm{A}\end{array}$
 

$\begin{array}{r}\mathrm{\Delta }\mathbf{V}\text{ from }\mathrm{A}\text{ to }\mathrm{P}\\ {\mathrm{V}}_{\mathrm{A}}-\frac{2}{50}\times 20={\mathrm{V}}_{\mathrm{P}}\end{array}$
$$\begin{gathered} \mathrm{\Delta V}\text{ from }\mathrm{A}\text{ to }\mathrm{Q} \\ {\mathrm{V}}_{\mathrm{A}}-\frac{2}{50}\times 30={\mathrm{V}}_{\mathrm{Q}} \\ {\mathrm{V}}_{\mathrm{P}}+\frac{2}{50}\times 20={\mathrm{V}}_{\mathrm{Q}}+\frac{2}{50}\times 30\Rightarrow {\mathrm{V}}_{\mathrm{P}}-{\mathrm{V}}_{\mathrm{Q}}=0.4\mathrm{V} \end{gathered}$$