Answer
Frequency is the reciprocal of period, and period is the reciprocal of frequency.
$$frequency = \frac{1}{period}$$
$$period = \frac{1}{frequency}$$
Work Step by Step
This makes sense. For example, if a guitar string vibrates at a frequency of 440 Hz, which is 440 times per second, it takes 1/440 of a second to complete one cycle.
This is discussed on page 359.