A pendulum takes 10 seconds to swing through 2 complete cycles what is it's period and frequency

Period = (10 seconds) / (2 swings)

Period = 5 seconds

Frequency = (2 swings) / (10 seconds)

Frequency = 0.2 per second. (0.2 Hertz)

Notice that ...

Period = 1 / Frequency,

and

(period) x (frequency) = 1.

These two things are true for every 'periodic' event ... anything that happens over and over and over and over and over. Like wave motion, your heart beat, or going to school.