There's a crest and a trough in each complete wave. So the question is describing 10 complete waves.
After that, the question becomes somewhat murky. It goes on to say "its time period is 0.2 seconds".
-- The "time period" of a wave is usually defined as the time for one complete wave. If that's what the phrase means, then ...
Frequency = ( 1/0.2sec )
Frequency = 5 Hz.
= = = = = = = = = =
BUT ... Is the question awkwardly trying to tell us that the 10 waves take 0.2 seconds ? If that's what it's saying, then ...
Frequency = (10) / (0.2 sec)
Frequency = 50 Hz .
