The period of an oscillation is the time to complete one cycle. Since the frequency is the number of cycles per second, it turns out that the period is the reciprocal of the frequency, f:
\(T=\frac{1}{f}\)
in this problem, the frequency of vibration of the string is f=20 Hz, therefore the period is
\(T=\frac{1}{f}=\frac{1}{20 Hz} =0.05 s\)
