A pendulum has a frequency of 0.80 Hz, calculate the periodic time of the pendulum.

You need to use the equation that relates frequency and period; period = 1/frequency

Then all you need to do is plug in the frequency to get the periodic time:

period = 1/0.80 Hz = 1.25 s

[NOTE: If you're confused as to how you get the unit of s from Hz, remember that another way of writing the unit Hz is s^-1, so when you divide by s^-1 it becomes just s.]