Verified Answer :

AL2006

Use your units . . . "period" is time, so you know that you have to
set up your fraction with time in the numerator.

                     10 seconds / 4 waves  = 

                         (10/4)  sec/wave  =  2.5 seconds per wave.
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Notice how the first answer set it up exactly backwards.
He would have seen his mistake immediately if he had
kept the units.

He started out by saying . . . .    P  =  N / t

Keeping the units,                  P  =  (waves) / (second)

The answer will have units of      "waves per second" .

That's frequency, not period.

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