In an experiment with sonometer when a mass of $180 \mathrm{~g}$ is attached to the string, it vibrates with fundamental frequency of $30 \mathrm{~Hz}$. When a mass $\mathrm{m}$ is attached, the string vibrates with fundamental frequency of $50 \mathrm{~Hz}$. The value of $\mathrm{m}$ is ___________ g.