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

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