In the circuit figure, the voltmeter reads 30 V. What is the resistance of the voltmeter?

The potential difference across $300\mathrm{\Omega }=60-30=30\mathrm{V}$
Therefore the effective resistance of voltmeter resistance R and 400 $\mathrm{\Omega }$ in parallel will be equal to 300 $\mathrm{\Omega }$, as 60 V is equally divided between two parts. So, $300=\frac{\mathrm{R}\times 400}{\mathrm{R}+400}$

or  $300\mathrm{R}+120000=400\mathrm{R}\text{ or }\mathrm{R}=1200\mathrm{\Omega }$