The reading of an ideal voltmeter in the circuit shown is 

Current from cell $=\frac{2}{25}$
P..D across $20\mathrm{\Omega } \mathrm{is}, 20\times \frac{1}{25}=\frac{20}{25}\mathrm{V}$
P.D across $30\mathrm{\Omega } \mathrm{is}, 30\times \frac{1}{25}=\frac{30}{25}\mathrm{v}$
∴ reading of volt meter $=\frac{10}{25}\mathrm{v}=0.4\mathrm{v}$