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