Frequency is the the number of wave oscillation that occur in a second. It is measured in Hertz (Hz).
In this question, the frequency of the note is the number of beats that occur in a second. Given that they are Thirty beats are heard in one minute, hence:
Frequency = 30 beat / 1 minute = 30 beats / 60 seconds = 0.5 beat per second = 0.5 Hz