Here's an interesting factoid to remember:
(Period of any wave) x (Frequency of the wave) = 1 .
If you can memorize that, then you'll always remember that
Period = 1 / frequency
and
Frequency = 1 / period .
The second one is what you need for this problem.
Frequency = 1 / period
= 1 / (4.85 x 10⁻² second)
= 1 / (0.0485 second)
= 20.62 / second
= 20.62 Hz. (rounded)
