The period T of a wave is the reciprocal of the frequency f:
[tex]T= \frac{1}{f} [/tex]
The frequency of the wave in the problem is [tex]f=700 Hz[/tex], so the period is (keeping in mind that [tex]1 Hz=1s^{-1}[/tex])
[tex]T= \frac{1}{700 Hz}= \frac{1}{700 s^{-1} }= \frac{1}{700} s [/tex]
