Asource of wave produces 20 crest sand 20 troughs in 0.4 sec. find the frequency of the wave.

1 whole wave is 1 crest and 1 trough.

20 crests and 20 troughs is 20 whole waves.

20 waves / 0.4 seconds = 50 waves per second.

Frequency = 50 Hz

see earlier answer

this is an example of "committed" time.

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