Question

If the period of the lowest-frequency sound you can hear is 0.050 s, then what is its frequency?

Answer 1

The frequency of a sound wave or any wave for that matter is the reciprocal of the period. That is,
                                            f = 1/T
Substituting the known value of period,
                                            f = 1/(0.05 s) 
                                            f = 20 Hz
Thus, the frequency of the wave is equal to 20 Hz.