Question

if one wave covers 20 crest and 20 through in 0.2 sec find frequency

Answer 1

Distance between two successive crusts or troughs is knowns as wavelength So Wavelength = 100 cm (because distance between crust & trough is 50 cm) In 0.2 seconds, it complete 20 waves in 1 second , it will complete 20 * 5 = 100 crests. So Frequency is 100 Hz.

Answer 2

20+20=40/2=20/2=10
10/0.2=50 hz