The relationship between frequency, wavelength and speed of a wave is: [tex]\lambda= \frac{v}{f} [/tex] where [tex]\lambda[/tex] is the wavelength v is the speed of the wave f is its frequency
For the sound wave in our problem, f=15 Hz and v=343 m/s, therefore the wavelength is [tex]\lambda= \frac{v}{f}= \frac{343 m/s}{15 Hz}=22.87 Hz [/tex]
