Chapter 4 - Graphs of the Circular Functions - Section 4.1 Graphs of the Sine and Cosine Functions - 4.1 Exercises - Page 146: 56b

20 cycles per second or 20 Hz

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.