Answer
20 cycles per second or 20 Hz
Work Step by Step
Frequency is the the number of cycles, or periods, completed in 1 second.
Since the period is $\frac{1}{20}$, one cycle is completed in $\frac{1}{20}$ second. Therefore, in one second, $20$ cycles are completed. This means that the frequency is $20$ cycles per second or $20$ Hz.