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